Several other large craters about 150 km wide and 7 km deep are also present. A dark feature about 200 km across has been named "Olbers" in honour of Vesta's discoverer, but it does not appear in elevation maps as a fresh crater would, and its nature is presently unknown, perhaps an old basaltic surface. It serves as a reference point with the 0° longitude prime meridian defined to pass through its center.