Europa

Europa is one of the many moons of Jupiter.

Its most interesting characteristic is the thick layer of ice covering a liquid water ocean. The water is kept from solidifying by the gravitational pull of Jupiter, causing heat to vent from the ocean floor. This is very similar to vents on Earth's own ocean floors, which are teeming with life.

The various temperatures ranging from warm to freezing would allow any potential life to avoid the warm-blooded size barrier brought on by overheating. Size would only be limited by the supporting ecosystem. Any potential life would also develop in complete darkness, leading to creatures very similar to our own deep sea organisms, like the anglerfish and viperfish.

In 1997, the U.S. National Oceanic and Atmospheric Administration (NOAA) detected an ultra-low frequency, extremely powerful underwater sound in the South Pacific Ocean. This sound is colloquially called the "Bloop". Its source remains unknown, but its profile is similar to that made by marine animals.

To make sound of this magnitude, the creature would have to be several times the size of a blue whale. This sound was recorded in waters that are just a couple miles deep. The ocean of Europa is believed to be 62 miles deep.