Concordia Subglacial Lake
Concordia Subglacial Lake is a subglacial lake located beneath an ice sheet 3,850 m (12,630 ft) to 4,125 m (13,533 ft) thick. It has a surface of about 800 km2 (310 sq mi) and is ~ 300 m (980 ft) deep.
Source: Wikipedia — Concordia Subglacial Lake (CC BY-SA 4.0)