For years, the mystery surrounding one of Saturn's ice-covered moons puzzled scientists.

The NASA Cassini spacecraft first observed a jet of water vapour spraying from the southern pole of the small moon Enceladus — hinting at a large body of water beneath the ice — back in 2005.

But how big is that body of water, and how does it not freeze over with such cold surface temperatures? Now, NASA has the answers, and Johanna Wagstaffe has the details. Watch the video above.