Here is one of the most peculiar craters on Mars. Taken by the Mars Global Surveyor, the photo clearly shows an ordinary-looking crater with something extraordinary-looking inside it. Off-center is a dome-like structure, nearly perfectly spherical, that appears to have a golf ball-like texture... or that of a Buckminster Fuller geodesic construction.
More intriguing, this dome stands very near what looks like a system of tunnels or conduits that run above and below the Martian surface. Small tunnels seem to connect to large tunnels, like some kind of enormous drainage system. (There is a similar crater and dome in this photo.)