The largest sea on Titan, the biggest moon of Saturn, fills most of this radar image from the Cassini spacecraft. Mare Kraken covers roughly twice the area of all the Great Lakes combined, and may be a thousand feet (300 meters) deep or more. Because Titan's surface is extremely cold, however, the sea is filled not with water, but with with liquid methane and ethane. Titan is covered by a thick orange haze, so scientists compiled this false-color image from observations by Cassini's radar, which could penetrate the haze. [NASA/JPL/Agenzia Spaziale Italiana/USGS]