According to the ESA on October 11, 2017, its : and Sentinel-1 missions have discovered huge canyons cutting through the underside of ice shelves in Antarctica.

The ice shelves act as conveyor belts, by which snow that falls in Antarctica ultimately makes its way back to the sea.

Portions of the ESA Statement: The ice sheet that covers Antarctica is, by its very nature, dynamic and constantly on the move. Recently, however, there has been a worrying number of reports about its floating shelves thinning and even collapsing, allowing the grounded ice inland to flow faster to the ocean and add to sea-level rise …

There are huge inverted canyons in the underside of ice shelves, but little is known about how they form and how they affect the stability of the ice sheet.

Secrets of Hidden Ice Canyons:  http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Secrets_of_hidden_ice_canyons_revealed/