Cassini Regio is the enigmatic dark area that covers half of Saturn's moon Iapetus. It is named after Giovanni Cassini, the discoverer of Iapetus; 'Regio' is a term used in planetary geology for a large area that is strongly differentiated in colour or albedo from its surroundings. The brighter half of Iapetus is called the Roncevaux Terra.
The nature of the material that caused the extensive discoloration of Cassini Regio is not known, but it appears to be quite thin. It may be the result of cryo-vulcanism, a spattering of material from outer moons, or residue left by the sublimation of brighter ice. The NASA Cassini probe flew by Iapetus on 2007 September, and the images are helping to clarify the nature of Cassini Regio. For more details, see the main article on Iapetus.
In 2004, it was discovered that a 1,300 km long ridge of 20 km (12 miles or 60,000 ft.) high mountains runs down the centre of Cassini Regio, almost perfectly following Iapetus's equator. The origin of this extremely unusual feature is as yet unknown.