Triton: Geysers at the Solar System's Edge
Though moons like Enceladus hog all the attention, Neptune's moon Triton gets no love despite having shooting geysers and active tectonics while orbiting at the very edge of the solar system. Triton's cryovolcanoes are also incredibly alien, spewing out nitrogen and keeping the surface free of any blemishing impact craters. Vast swathes of the moon are also covered in weird, mottled plains resembling cantaloupe peel.
No one exactly understands how Triton's active volcanism works, especially considering that its surface temperature is only slightly above absolute zero. Instead of molten rock, water and ammonia flow over its face, carving complex valleys and ridges. The bizarre cantaloupe terrain is completely unlike anything else in the solar system, and is thought to form when lumps of material rise upward through a layer of denser surface.
Only one probe, Voyager 2, has ever ventured anywhere near the frozen moon.