Question: While the Sydney Harbour Bridge is a steel through arch bridge, the Sydney Harbour Tunnel is what?

Context paragraphs:
Sydney Harbour Bridge: The Sydney Harbour Bridge is a steel through arch bridge across Sydney Harbour that carries rail, vehicular, bicycle, and pedestrian traffic between the Sydney central business district (CBD) and the North Shore.  The dramatic view of the bridge, the harbour, and the nearby Sydney Opera House is an iconic image of Sydney, and Australia.  The bridge is nicknamed "The Coathanger" because of its arch-based design.

Lower North Shore (Sydney): The Lower North Shore refers to the Sydney Harbour Peninsula that is located on the north side of the Sydney Harbour Bridge.  The three bodies of water that surround the Lower North Shore are Lane Cove River on its western boarder, Sydney harbour on its south side, and Middle Harbour on its east.  The Lower North Shore borders the Upper North Shore when the Lane Cover River and Middle Harbour are at their closest.

Jeffrey Street: Jeffrey Street or Jeffreys Street is a street located in Kirribilli, famous for being one of the most popular vantage points for views of the city skyline of Sydney, the Sydney Harbour Bridge and the Sydney Opera House.  The street is located on the Lower North Shore of Sydney Harbour, directly across the harbour from Circular Quay and is a popular destination for tourists, particularly on Australia Day and New Year's Eve.  The street leads uphill from the harbour in a northerly direction to the small shopping village of Kirribilli.

Sydney Harbour Bridge cycleway: The Sydney Harbour Bridge cycleway is a 2 km conjoined cycleway from the north and south that crosses the Sydney Harbour Bridge on its western side, linking the Sydney central business district with North Sydney, Sydney's Northern Suburbs and the North Shore, in Sydney, New South Wales, Australia.

Clark Island (New South Wales): Clark Island is a small island in Sydney Harbour, near the coast of New South Wales, Australia.  The island is part of the Sydney Harbour National Park and lies offshore the Sydney suburb of Darling Point, in the eastern part of Sydney Harbour between the Harbour Bridge and the harbour entrance.

Gladesville Bridge: Gladesville Bridge is an arch bridge near Gladesville that spans the Parramatta River, west of central Sydney, Australia.  It links the suburbs of Huntleys Point and Drummoyne.  It is a few kilometres upstream of the more famous Sydney Harbour Bridge and is part of Victoria Road.  When it was completed in 1964, Gladesville Bridge was the longest single span concrete arch ever constructed.  Gladesville Bridge is the largest of a complex of three bridges, including Fig Tree Bridge and Tarban Creek Bridge, designed to carry traffic as part of the North Western Expressway.  The bridge was the first phase of this freeway project that was to connect traffic from the Newcastle via Wahroonga/Lane Cove, then through Glebe/Annandale to connect into the city.  Due to community action the freeway project was abandoned by the Wran government in 1977, leaving the Gladesville bridge connecting the existing arterial roads.

Sydney Harbour Tunnel: The Sydney Harbour Tunnel is a twin-tube road tunnel in Sydney, Australia.  The tunnel was completed and opened to traffic in August 1992 to provide a second vehicular crossing of Sydney Harbour to alleviate congestion on the Sydney Harbour Bridge.  It is the only tunnel under the harbour, although there are plans to build a set of rail tunnels.

Admiralty House, Sydney: Admiralty House is the official Sydney residence of the Governor-General of Australia.  It is located in the suburb of Kirribilli, on the northern foreshore of Sydney Harbour (adjacent to Kirribilli House, which is the Sydney residence of the Australian Prime Minister).  This large, Italianate, sandstone mannor occupies the tip of Kirribilli Point.  Once known as "Wotonga", it has commanding views across Sydney Harbour to the Sydney Harbour Bridge and the Sydney Opera House.

Port Jackson: Port Jackson, consisting of the waters of Sydney Harbour, Middle Harbour, North Harbour and the Lane Cove and Parramatta Rivers, is the ria or natural harbour of Sydney, New South Wales, Australia.  The harbour is an inlet of the Tasman Sea (part of the South Pacific Ocean).  It is the location of the Sydney Opera House and Sydney Harbour Bridge.  The location of the first European settlement in Australia, Port Jackson has continued to play a key role in the history and development of Sydney.

North Sydney Olympic Pool: The North Sydney Olympic Pool is a swimming and exercise complex located adjacent to Sydney Harbour at Milsons Point in North Sydney between the Sydney Harbour Bridge and Luna Park.  Designed by architects Rudder & Grout in the Inter-War Free Classical style with art deco-style decorations, the Olympic-sized outdoor pool was built on part of the Dorman Long workshops site following the completion of the Harbour Bridge.  The pool opened 4 April 1936 and hosted the swimming and diving events for the 1938 Empire Games.  Heating was added in 2000 and a 25m indoor pool was added in 2001.  Despite the name, it was not the aquatic venue for the 2000 Sydney Olympic Games; the aquatic events took place at the Sydney International Aquatic Centre in Sydney Olympic Park.

Supporting paragraphs: Sydney Harbour Tunnel Sydney Harbour Bridge
Answer: a twin-tube road tunnel