Document:

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                                                                     Exhibit 4.9

                           SPLITROCK SERVICES, INC.,

                                  as Parent,

                            SPLITROCK LEASING, LLC,

                           as Subsidiary Guarantor,

                                      and

                       HARRIS TRUST COMPANY OF NEW YORK

                                  as Trustee

              __________________________________________________

                         SECOND SUPPLEMENTAL INDENTURE

                         Dated as of December 22, 1999

                                 to INDENTURE

                           Dated as of July 24, 1998

              __________________________________________________

                                 $261,000,000

                         11 3/4% Senior Notes due 2008
<PAGE>

     SECOND SUPPLEMENTAL INDENTURE (this "Supplemental Indenture") dated as of
December 22, 1999, by and among SPLITROCK SERVICES, INC., a Delaware corporation
(the "Company"), SPLITROCK LEASING LLC, a Delaware limited liability company
(the "Subsidiary Guarantor"), and HARRIS TRUST COMPANY OF NEW YORK (formerly,
BANK OF MONTREAL TRUST COMPANY, a New York banking corporation, as trustee under
the indenture referred to below (the "Trustee").

                                  WITNESSETH
                                  ----------

     WHEREAS the Company has heretofore executed and delivered to the Trustee an
Indenture dated as of July 24, 1998, providing for the issuance of an aggregate
principal amount of up to $311,000,000 of 113/4% Senior Notes due 2008 (the
"Securities") and a First Supplemental Indenture, dated November 24, 1999
(together, the "Indenture"),  implementing certain amendments described in a
Consent Solicitation Statement, dated November 9, 1999 as supplemented by a
Supplement thereto, dated November 19, 1999;

     WHEREAS pursuant to Section 4.11 of the Indenture, the Subsidiary
Guarantor, the Company and the Trustee are required to execute and deliver this
Supplemental Indenture pursuant to which the Subsidiary Guarantor shall
unconditionally guarantee all the Company's obligations under the Indenture and
the Securities pursuant to a Subsidiary Guarantee on the terms and conditions
set forth herein;

     WHEREAS, this Supplemental Indenture has been duly authorized by all
necessary corporate action on the part of the Company; and

     WHEREAS, the Company has delivered, or caused to be delivered, to the
Trustee, an Officer's Certificate and an Opinion of Counsel meeting the
requirements of Section 11.04 and 11.05 of the Indenture and stating that all
conditions precedent and covenants, if any, provided for in the Indenture
relating to this Supplemental Indenture have been satisfied.

     NOW THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, receipt of which is hereby acknowledged, the Subsidiary
Guarantor, the Company, and the Trustee mutually covenant and agree for the
equal and ratable benefit of the holders of the Securities as follows:

                                   ARTICLE I
                                   ---------

                       Addition of Subsidiary Guarantor
                       --------------------------------

     SECTION 1.01. Subsidiary Guarantee.  The Subsidiary Guarantor hereby
                   ---------------------
jointly and severally unconditionally and irrevocably guarantees, as a primary
obligor and not merely as a surety, to each Holder and to the Trustee and its
successors and assigns (a) the full and punctual payment of principal of and
interest on and liquidated damages in respect of the Securities when due,
whether at Stated Maturity, by acceleration, by redemption or otherwise, and all
other monetary obligations of the Company under the Indenture or this
Supplemental Indenture (including obligations to the Trustee) and the Securities
and (b) the full and punctual performance within applicable grace periods of all
other obligations of the Company whether for

                                       2
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expenses, indemnification or otherwise under the Indenture, this Supplemental
Indenture and the Securities (all the foregoing being hereinafter collectively
called the "Guaranteed Obligations"). The Subsidiary Guarantor further agrees
that the Guaranteed Obligations may be extended or renewed, in whole or in part,
without notice or further assent from the Subsidiary Guarantor, and that the
Subsidiary Guarantor shall remain bound under this Article I notwithstanding any
extension or renewal of any Guaranteed Obligation.

     The Subsidiary Guarantor waives presentment, demand of, payment from and
protest to the Company of any of the Guaranteed Obligations and also waives
notice of protest for nonpayment.  The Subsidiary Guarantor waives notice of any
default under the Securities or the Guaranteed Obligations.  The obligations of
the Subsidiary Guarantor hereunder shall not be affected by (a) the failure of
any Holder or the Trustee to assert any claim or demand or to enforce any right
or remedy against the Company or any other Person under the Indenture or this
Supplemental Indenture, the Securities or any other agreement or otherwise; (b)
any extension or renewal of any thereof; (c) any rescission, waiver, amendment
or modification of any of the terms or provisions of this Indenture, the
Securities or any other agreement; (d) the release of any security held by any
Holder or the Trustee for the Guaranteed Obligations or any of them; (e) the
failure of any Holder or Trustee to exercise any right or remedy against any
other guarantor of the Guaranteed Obligations; or (f) any change in the
ownership of such Subsidiary Guarantor, except as provided in Section 1.02(b).

     The Subsidiary Guarantor hereby waives any right to which it may be
entitled to have its obligations hereunder divided among any other Subsidiary
Guarantors, such that such Subsidiary Guarantor's obligations would be less than
the full amount claimed.  The Subsidiary Guarantor hereby waives any right to
which it may be entitled to have the assets of the Company first be used and
depleted as payment of the Company's or such Subsidiary Guarantor's obligations
hereunder prior to any amounts being claimed from or paid by such Subsidiary
Guarantor hereunder.  The Subsidiary Guarantor hereby waives any right to which
it may be entitled to require that the Company be sued prior to an action being
initiated against such Subsidiary Guarantor.  The Subsidiary Guarantor waives
and will not in any manner whatsoever claim or take the benefit or advantage of,
any rights of reimbursement, indemnity or subrogation or any other rights
against the Company or any other Subsidiary Guarantor as a result of any payment
by such Subsidiary Guarantor under its Subsidiary Guarantee.

     The Subsidiary Guarantor further agrees that its Subsidiary Guarantee
herein constitutes a guarantee of payment, performance and compliance when due
(and not a guarantee of collection) and waives any right to require that any
resort be had by any Holder or the Trustee to any security held for payment of
the Guaranteed Obligations.

     Except as expressly set forth in Section 8.01(b) of the Indenture and
Section 1.02 of this Supplemental Indenture, the obligations of the Subsidiary
Guarantor hereunder shall not be subject to any reduction, limitation,
impairment or termination for any reason, including any claim of waiver,
release, surrender, alteration or compromise, and shall not be subject to any
defense of setoff, counterclaim, recoupment or termination whatsoever or by
reason of the invalidity, illegality or unenforceability of the Guaranteed
Obligations or otherwise.  Without limiting the generality of the foregoing, the
obligations of the Subsidiary Guarantor under the

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Indenture or this Supplemental Indenture shall not be discharged or impaired or
otherwise affected by the failure of any Holder or the Trustee to assert any
claim or demand or to enforce any remedy under the Indenture or this
Supplemental Indenture, the Securities or any other agreement, by any waiver or
modification of any thereof, by any default, failure or delay, wilful or
otherwise, in the performance of the obligations, or by any other act or thing
or omission or delay to do any other act or thing which may or might in any
manner or to any extent vary the risk of any Subsidiary Guarantor or would
otherwise operate as a discharge of any Subsidiary Guarantor as a matter of law
or equity.

     The Subsidiary Guarantor agrees that its Subsidiary Guarantee shall remain
in full force and effect until payment in full of all the Guaranteed
Obligations. The Subsidiary Guarantor further agrees that its Subsidiary
Guarantee herein shall continue to be effective or be reinstated, as the case
may be, if at any time payment, or any part thereof, of principal of or interest
on any Guaranteed Obligation is rescinded or must otherwise be restored by any
Holder or the Trustee upon the bankruptcy or reorganization of the Company or
otherwise.

     In furtherance of the foregoing and not in limitation of any other right
which any Holder or the Trustee has at law or in equity against any Subsidiary
Guarantor by virtue hereof, upon the failure of the Company to pay the principal
of or interest on any Guaranteed Obligation when and as the same shall become
due, whether at maturity, by acceleration, by redemption or otherwise, or to
perform or comply with any other Guaranteed Obligation, the Subsidiary Guarantor
hereby promises to and shall, upon receipt of written demand by the Trustee,
forthwith pay, or cause to be paid, in cash, to the Holders or the Trustee an
amount equal to the sum of (i) the unpaid principal amount of such Guaranteed
Obligations, (ii) accrued and unpaid interest on such Guaranteed Obligations
(but only to the extent not prohibited by law) and (iii) all other monetary
obligations of the Company to the Holders and the Trustee.

     The Subsidiary Guarantor agrees that it shall not be entitled to any right
of subrogation in relation to the Holders in respect of any Guaranteed
Obligations guaranteed hereby until payment in full of all Guaranteed
Obligations. The Subsidiary Guarantor further agrees that, as between it, on the
one hand, and the Holders and the Trustee, on the other hand, (x) the maturity
of the Guaranteed Obligations guaranteed hereby may be accelerated as provided
in Article 6 of the Indenture, notwithstanding any stay, injunction or other
prohibition preventing such acceleration in respect of the Guaranteed
Obligations guaranteed hereby, and (y) in the event of any declaration of
acceleration of such Guaranteed Obligations as provided in Article 6 of the
Indenture, such Guaranteed Obligations (whether or not due and payable) shall
forthwith become due and payable by such Subsidiary Guarantor for the purposes
of this Section 1.01.

     The Subsidiary Guarantor also agrees to pay any and all costs and expenses
(including reasonable attorneys' fees and expenses) incurred by the Trustee or
any Holder in enforcing any rights under this Section 1.01.

     Upon request of the Trustee, the Subsidiary Guarantor shall execute and
deliver such further instruments and do such further acts as may be reasonably
necessary or proper to carry out more effectively the purpose of the Indenture
or this Supplemental Indenture.

                                       4
<PAGE>

     SECTION 1.02. Limitation on Liability.  (a) Any term or provision of this
                   ------------------------
Supplemental Indenture or the Indenture to the contrary notwithstanding, the
maximum, aggregate amount of the Guaranteed Obligations guaranteed hereunder by
the Subsidiary Guarantor shall not exceed the maximum amount that can be hereby
guaranteed without rendering the Indenture or this Supplemental Indenture, as it
relates to such Subsidiary Guarantor, voidable under applicable law relating to
fraudulent conveyance or fraudulent transfer or similar laws affecting the
rights of creditors generally.

     (b)  This Subsidiary Guarantee shall terminate and be of no further force
or effect and the Subsidiary Guarantor shall be deemed to be released from all
obligations under this Article I upon (i) the merger or consolidation of the
Subsidiary Guarantor with or into any Person other than the Company or a
Subsidiary or Affiliate of the Company where the Subsidiary Guarantor is not the
surviving entity of such consolidation or merger or (ii) the sale by the Company
or any Subsidiary of the Company (or any pledgee of the Company) of the Capital
Stock of the Subsidiary Guarantor, where, after such sale, such Subsidiary
Guarantor is no longer a Subsidiary of the Company; provided, however, that each
                                                    --------  -------
such merger, consolidation or sale (or, in the case of a sale by such a pledgee,
the disposition of the proceeds of such sale) shall comply with Sections 4.06
and 5.01(b) of the Indenture. At the request of the Company, the Trustee shall
execute and deliver an appropriate instrument evidencing such release.

     SECTION 1.03. Successors and Assigns.  This Article I shall be binding upon
                   -----------------------
the Subsidiary Guarantor and its successors and assigns and shall inure to the
benefit of the successors and assigns of the Trustee and the Holders and, in the
event of any transfer or assignment of rights by any Holder or the Trustee, the
rights and privileges conferred upon that party in the Indenture and this
Supplemental Indenture and in the Securities shall automatically extend to and
be vested in such transferee or assignee, all subject to the terms and
conditions of the Indenture and this Supplemental Indenture.

     SECTION 1.04. No Waiver.  Neither a failure nor a delay on the part of
                   ----------
either the Trustee or the Holders in exercising any right, power or privilege
under this Article I shall operate as a waiver thereof, nor shall a single or
partial exercise thereof preclude any other or further exercise of any right,
power or privilege.  The rights, remedies and benefits of the Trustee and the
Holders in the Indenture and this Supplemental Indenture expressly specified are
cumulative and not exclusive of any other rights, remedies or benefits which
either may have under this Article I at law, in equity, by statute or otherwise.

     SECTION 1.05. Modification.  No modification, amendment or waiver of any
                   -------------
provision of this Article I, nor the consent to any departure by the Subsidiary
Guarantor therefrom, shall in any event be effective unless the same shall be in
writing and signed by the Trustee, and then such waiver or consent shall be
effective only in the specific instance and for the purpose for which given.  No
notice to or demand on the Subsidiary Guarantor in any case shall entitle such
Subsidiary Guarantor to any other or further notice or demand in the same,
similar or other circumstances.

                                  ARTICLE II

                                 Miscellaneous
                                 -------------

                                       5
<PAGE>

     SECTION 2.01.  Ratification of Indenture: Supplemental Indentures Part of
                    ----------------------------------------------------------
Indenture.  Except as expressly amended hereby, the Indenture is in all respects
----------
ratified and confirmed and all the terms, conditions and provisions thereof
shall remain in full force and effect. Upon the execution and delivery of this
Supplemental Indenture by the Company and the Trustee, this Supplemental
Indenture shall form a part of the Indenture for all purposes, and every holder
of Securities heretofore or hereafter authenticated and delivered shall be bound
hereby. Any and all references, whether within the Indenture or in any notice,
certificate or other instrument or document, shall be deemed to include a
reference to this Supplemental Indenture (whether or not made), unless the
context shall otherwise require.

     SECTION 2.02.  Governing Law; Governance, Etc. THIS SUPPLEMENTAL INDENTURE
                    -------------------------------
SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF
NEW YORK BUT WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW
TO THE EXTENT THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE
REQUIRED THEREBY. This Supplemental Indenture shall be governed and construed in
accordance with the applicable terms and provisions of the Indenture as amended
hereby, which terms and provisions are incorporated herein by reference, as if
this Supplemental Indenture were the "Indenture" referred to therein.

     SECTION 2.03.  Trustee Makes No Representation.  The Trustee makes no
                    --------------------------------
representation as to the validity or sufficiency of this Supplemental Indenture.

     SECTION 2.04.  Counterparts.  The parties may sign any number of copies of
                    -------------
this Supplemental Indenture.  Each signed copy shall be an original, but all of
them together represent the same agreement.

     SECTION 2.05.  Effect of Headings.  The Section headings herein are for
                    -------------------
convenience only and shall not effect the construction thereof

     SECTION 2.06.  Terms.  Certain capitalized terms used but not defined
                    ------
herein have the meanings assigned to them in the Indenture.

                                       6
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Second Supplemental
Indenture to be duly executed as of the date first above written.

                                        SPLITROCK SERVICES, INC.

                                        By_____________________________________
                                        Name:
                                        Title:

                                        SPLITROCK LEASING LLC

                                        By _____________________________________
                                        Name:
                                        Title:

                                        HARRIS TRUST COMPANY OF NEW YORK,
                                        as Trustee,

                                        By ____________________________________
                                        Name:
                                        Title:

                                       7<PAGE>

                                                                    EXHIBIT 4.10

                           SPLITROCK SERVICES, INC.,

                                   as Parent

                            SPLITROCK LEASING, LLC

                            as Subsidiary Guarantor

                                      and

                       HARRIS TRUST COMPANY OF NEW YORK

                                  as Trustee

              __________________________________________________

                         THIRD SUPPLEMENTAL INDENTURE

                         Dated as of February 11, 2000

                                 to INDENTURE

                          Dated as of July 24, 1998,
                               as amended by the
                 First and the Second Supplemental Indentures

              __________________________________________________

                                 $261,000,000

                         11 3/4% Senior Notes due 2008
<PAGE>

     THIRD SUPPLEMENTAL INDENTURE, dated as of February 11, 2000, by and between
SPLITROCK SERVICES, INC., a Delaware corporation, (the "Company"), SPLITROCK
LEASING, LLC, a Delaware limited liability company (the "Subsidiary Guarantor")
and HARRIS TRUST COMPANY OF NEW YORK (formerly BANK OF MONTREAL TRUST COMPANY),
a New York trust company, as Trustee (the "Trustee").

                                  WITNESSETH
                                  ----------

     WHEREAS the Company and the Subsidiary Guarantor have heretofore executed
and delivered to the Trustee an Indenture dated as of July 24, 1998, as amended
by the First and the Second Supplemental Indentures (as amended, the
"Indenture"), providing for the issuance of an aggregate principal amount of up
to $311,000,000 of 11 3/4% Senior Notes due 2008 (the "Securities");

     WHEREAS, Section 9.01(a) of the Indenture provides, among other things,
that the Company, the Subsidiary Guarantor and the Trustee may amend the
Indenture without the consent of the holders of the Securities in order to cure
any ambiguity, omission, defect or inconsistency;

     WHEREAS, the Company and the Trustee desire to amend Section 5.01 of the
Indenture to cure an ambiguity therein;

     WHEREAS, this Third Supplemental Indenture has been duly authorized by all
necessary corporate action on the part of the Company and the Subsidiary
Guarantor; and

     WHEREAS, the Company has delivered, or caused to be delivered, to the
Trustee, an Officer's Certificate and an Opinion of Counsel meeting the
requirements of Section 11.04 and 11.05 of the Indenture and stating that all
conditions precedent and covenants, if any, provided for in the Indenture
relating to this Third Supplemental Indenture have been satisfied.

     NOW THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, receipt of which is hereby acknowledged, the Company,
the Subsidiary Guarantor, and the Trustee mutually covenant and agree for the
equal and ratable benefit of the holders of the Securities as follows:

                                       1
<PAGE>

                                   ARTICLE I
                                   ---------

                                   Amendment
                                   ---------

     SECTION 1.01.  Amendment to Indenture. The Indenture is hereby amended as
                    ----------------------
follows:

          By deleting the last paragraph of Section 5.01 and replacing it with
     the following new subsection (c):

          "(c) Notwithstanding anything contained in this Section 5.01, (i) any
     Restricted Subsidiary may consolidate with, merge into or transfer all or
     part of its properties and assets to the Company and (ii) the Company may
     merge with an Affiliate incorporated solely for the purpose of
     reincorporating the Company in another jurisdiction to realize tax or other
     benefits."

                                  ARTICLE II
                                  ----------

                                 Miscellaneous
                                 -------------

     SECTION 2.01.  Ratification of Indenture: Supplemental Indentures Part
                    -------------------------------------------------------
of Indenture. Except as expressly amended hereby, the Indenture is in all
-------------
respects ratified and confirmed and all the terms, conditions and provisions
thereof shall remain in full force and effect. Upon the execution and delivery
of this Third Supplemental Indenture by the Company and the Trustee, this Third
Supplemental Indenture shall form a part of the Indenture for all purposes, and
every holder of Securities heretofore or hereafter authenticated and delivered
shall be bound hereby. Any and all references, whether within the Indenture or
in any notice, certificate or other instrument or document, shall be deemed to
include a reference to this Third Supplemental Indenture (whether or not made),
unless the context shall otherwise require.

     SECTION 2.02.  Governing Law; Governance, Etc. THIS THIRD SUPPLEMENTAL
                    -------------------------------
INDENTURE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF
THE STATE OF NEW YORK BUT WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF
CONFLICTS OF LAW TO THE EXTENT THAT THE APPLICATION OF THE LAWS OF ANOTHER
JURISDICTION WOULD BE REQUIRED THEREBY. This Third Supplemental Indenture shall
be governed and construed in accordance with the applicable terms and provisions
of the Indenture as amended hereby, which terms and provisions are incorporated
herein by reference, as if this Third Supplemental Indenture were the
"Indenture" referred to therein.

     SECTION 2.03.  Trustee Makes No Representation. The Trustee makes no
                    --------------------------------
representation as to the validity or sufficiency of this Third Supplemental
Indenture.

     SECTION 2.04.  Counterparts. The parties may sign any number of copies of
                    -------------
this Third Supplemental Indenture.  Each signed copy shall be an original, but
all of them together represent the same agreement.

                                       2
<PAGE>

     SECTION 2.05.  Effect of Headings. The Section headings herein are for
                    ------------------
convenience only and shall not affect the construction thereof

     SECTION 2.06.  Terms. Certain capitalized terms used but not defined
                    -----
herein have the meanings assigned to them in the Indenture.

     SECTION 2.07.  Entire Agreement. This Third Supplemental Indenture,
                    ----------------
together with the Indenture as amended hereby and the Securities, contains the
entire agreement of the parties, and supersedes all other representations,
warranties, agreements and understandings between the parties, oral or
otherwise, with respect to the matters contained herein and therein.

     SECTION 2.08.  Benefits of Third Supplemental Indenture. Nothing in this
                    ----------------------------------------
Third Supplemental Indenture, the Indenture, or the Securities, express or
implied, shall give to any Person, other than the parties hereto and thereto and
their successors hereunder and thereunder, and the Securityholders, any benefit
of any legal or equitable right, remedy or claim under the Indenture, the Third
Supplemental Indenture or the Securities.

                                       3
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Third Supplemental
Indenture to be duly executed as of the date first above written.

                                   SPLITROCK SERVICES, INC.

                                   By_____________________________________
                                   Name: William R. Wilson
                                   Title: President and Chief Executive Officer

                                   SPLITROCK LEASING, LLC,
                                   as Subsidiary Guarantor

                                   By_____________________________________
                                   Name: William R. Wilson
                                   Title: Authorized Representative

                                   HARRIS TRUST COMPANY OF NEW YORK,
                                   as Trustee,

                                   By ____________________________________
                                   Name:
                                   Title:

                                       4

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