I absolutely love that there's water beneath glaciers, that's always such a cool thought.

There are even subglacial lakes, and they contain 15% of the world's liquid freshwater.

The largest is Lake Vostok in antarctica. By volume, lake Vostok is the 6th largest lake in the world. The surface of this lake is about 4000m (13,100ft) beneath the ice. It's potentially been isolated from the rest of Earth's ecosystem for 15-25M years. So if there is any life there, then it'll be unlike anything we've ever seen.

It's also believed to be a similar environment to what we'd find in the subsurface oceans if Enceladus and Europa. So it could be used to test exploration techniques before we travel there.

They remain liquid, because the downward pressure of the ice decreases the melting point into a range that geothermal heat can melt it.