A downward-projecting ridge on the underside of the ice canopy; the submerged counterpart of a ridge. Ice keels may extend as much as 50 metres below the surface.
Environment and Climate Change - Canada https://www.ec.gc.ca/glaces-ice/default.asp?lang=En&n=2CE448E2%E2%80%931&offset=4&toc=hide

By Lusilier - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=65555340