A locking mechanism for locking an insertable device relative to an outer tube of a ground drill, the locking mechanism comprising: at least one latch, which is arranged to releasably retain the insertable device in a substantially fixed axial position relative to the outer tube, wherein the locking mechanism comprises a cam and a cam follower, which are arranged to control a radial position of the latch, the latch being provided, at a first longitudinal edge thereof, with an engagement surface adapted to engage a latch seat, and presenting a second longitudinal edge, opposite said first longitudinal edge, the latch being pivotable about a pivot axis at a first axial end of the latch, and the cam being arranged at an axially substantially opposite second axial end of the latch, wherein the cam presents a withdrawal surface against which the cam follower is arranged to slide to cause the latch to withdraw to unlock the insertable device from the outer tube, the withdrawal surface extending from a first end (21 b) of the cam to a second end (21 a) of the cam, the first end representing a distal portion and the second end representing a proximal portion of the cam and wherein the cam follower is movable along the withdrawal surface from the first end of the cam, closest to the pivot axis to the second end of the cam, farthest away from the pivot axis, wherein the withdrawal surface at the second end of the cam is closer to the first longitudinal edge than at any other point along the cam; characterised in that: the withdrawal surface is contiguously curved between the distal portion and the proximal portion of the cam. at least one latch, which is arranged to releasably retain the insertable device in a substantially fixed axial position relative to the outer tube, wherein the locking mechanism comprises a cam and a cam follower, which are arranged to control a radial position of the latch, the latch being provided, at a first longitudinal edge thereof, with an engagement surface adapted to engage a latch seat, and presenting a second longitudinal edge, opposite said first longitudinal edge, the latch being pivotable about a pivot axis at a first axial end of the latch, and the cam being arranged at an axially substantially opposite second axial end of the latch, wherein the cam presents a withdrawal surface against which the cam follower is arranged to slide to cause the latch to withdraw to unlock the insertable device from the outer tube, the withdrawal surface extending from a first end (21 b) of the cam to a second end (21 a) of the cam, the first end representing a distal portion and the second end representing a proximal portion of the cam and wherein the cam follower is movable along the withdrawal surface from the first end of the cam, closest to the pivot axis to the second end of the cam, farthest away from the pivot axis, wherein the withdrawal surface at the second end of the cam is closer to the first longitudinal edge than at any other point along the cam; characterised in that: the withdrawal surface is contiguously curved between the distal portion and the proximal portion of the cam. the withdrawal surface is contiguously curved between the distal portion and the proximal portion of the cam.