https://phys.org/news/2018-05-nasa-deep-life.html

Saturn's moon Enceladus and Jupiter's moon Europa are thought to have liquid oceans and hydrothermal activity under icy crusts. Locations on Earth with key similarities to future deep-space destinations are called analog environments. SUBSEA's target, the springs emerging from a volcano forming the next Hawaiian island, called the Lō`ihi seamount, is an analog for these ocean worlds.

When NASA's Cassini mission to Saturn discovered a plume of water erupting from beneath the icy surface of Enceladus, the characteristics of the plume told scientists what conditions might be like on the sea floor. This included the temperature, pressure and composition, and suggested the presence of hydrothermal activity. Scientists think these moons are good places to look for potential life, because water interacting with rock on their sea floors could yield chemical reactions that would make microbial metabolism possible.