Mercury is so close to the Sun that it was all but impossible to learn much about this planet until the Mariner spacecraft visited this harsh world. Mariner 10 visited Mercury three times in 1974-5. It showed a heavily cratered world, very similar to our Moon. There were large ancient lava flows that had been subsequently cratered.

     Just opposite Caloris basin is some very odd looking terrain, which is, not surprisingly called weird terrain! It does indeed look "weird." This area is multi-grooved with smooth terrain between the grooves. It is believed that this terrain was caused by shock waves resulting from the impact that formed Caloris basin.

 One type of terrain common to Mercury is lobate scarps. Thiese curved cliffs stretch more than 500 km each accross the surface. They cut through most existing craters, indicating that they are formed more recently in Mercury's history. Apparently they formed when Mercury cooled and the crust shrank and then wrinkled into these curving scarps.