According to an analysis by a team of planetary scientists headed by James Dohm (University of Arizona), a large, underground aquifer lies within the remains of an ancient impact basin in Arabia Terra on Mars. Specifically, the team proposes that the Arabia basin spanned 1,800 miles (3,000 km) in diameter, and that it formed sometime before about 3.5 billion years ago. As essentially a big hole in the ground, such a place would have attracted and trapped a lot of water and sediments during much of Mars’s history. The team has combined data from spacecraft in Mars orbit with geophysical arguments to build what is admittedly a circumstantial case.