An ancient crater near Mars' equator. Hundreds of "mud volcanoes", mounds up to 500 meters across and dozens high, have been found. These are thought to have formed when the surface was wetter from material that oozed out to the surface. Not formed from molten rock as there are no deposits of volcanic ash or lava nearby.
Firsoff Crater Web Pages
Firsoff Crater In the News
First Mud Volcanoes Linked to Underlying Sediments Found (Apr 2011)