A cold, pockmarked, Mercury-sized rock orbiting 1.2 million miles from Jupiter, Callisto would appear at first glance to be a strange candidate for extraterrestrial life—but as we look at the possibility of exploring underground oceans on the Jovian moons of Europa and Ganymede, we'd be well served to remember that Callisto comes from the same neighborhood, already has visible surface ice, and may contain mineral-rich underground oceans of its own.
You can see the full rotation of Callisto in this gorgeous computer animation from Kurdistan Planetarium:
The case against Callisto is the conspicuous lack of geological activity, which removes one of the possible driving forces behind chemosynthesis. But the case for Callisto is that when NASA's Human Outer Planets Exploration (HOPE) initiative looked at sites in the outer solar system for possible human colonization, they looked at Callisto—and they liked what they saw. The moon's distance from Jupiter means low radiation, and the relative geological inactivity means it's safer for human colonists. Could these same attributes promote, in some not-yet-understood way, the evolution of organisms under the planet's icy surface?
We're not likely to know for some time; even Europa, long considered a more likely candidate for extraterrestrial life, is not high on NASA's list of priorities. But as CNSA, ISRO, the ESA, JAXA, and private companies begin to take on ambitious projects of their own, the possibility of a mission to explore the subsurface of Callisto looks increasingly plausible.