An ocean under the ice. Under Europa’s cracked ice lies a salty ocean holding about twice as much water as all the Earth’s seas. NASA’s Europa Clipper reaches it in 2030.
A mosaic of Galileo and Voyager pictures, in black and white. Near the poles no spacecraft has looked closely, so those parts are plain grey.
USGS Astrogeology, Galileo and Voyager global mosaic, public domain.