Formed or deposited beneath a glacier. When an ice sheet flows over unfrozen sediment, it may cause subglacial bed deformation, as the water in the sediment pores increases enough to reduce the resistance between the individual grains. Subglacial water flows are maintained by high water pressure, rather than by melting of the subglacial channel walls. See Siegert (2005) Ann. Rev. Earth & Plan. Sci. 33 and Boulton et al. (2007) Qly. Sci. Rev. 26, 7–8. More than 70 subglacial lakes exist in Antarctica; the largest, L. Vostok, lies beneath 3 km of ice and is 230 km long—see M. M. Bennett and N. Glasser (2009).