Final answer:
Europa, Jupiter's moon, is likely to host a deep subsurface ocean, making it of great interest to those searching for extraterrestrial life due to the essential nature of liquid water for life.
Step-by-step explanation:
The moon considered likely to have a deep, subsurface ocean of liquid water is Europa. Europa, which orbits Jupiter, has an icy surface that is believed to conceal an ocean with a depth of tens to perhaps a hundred kilometers. The moon's orbit around Jupiter results in gravitational forces creating tides, similar to Earth's ocean tides, and the subsequent friction generates enough heat to keep the water beneath its icy crust in liquid form. This phenomenon presents a tantalizing environment for astrobiologists searching for extraterrestrial life, as liquid water is essential for life as we know it.