What accounts for the low-lying, flat surface of Mars's north? On Earth's surface, higher- and lower-lying areas have different types of crust: one, thin and dense, is pulled toward Earth's center more strongly by gravity, and the planet's water naturally comes to sit over it, creating oceans. The processes that generate this oceanic crust drive plate tectonics. Is Mars's north similarly characterized by a sort of crust different from other areas of the planet? Some researchers do see signs of tectonic activity surrounding the northern basin that suggest that it was created through the formation of new crust, like ocean basins on Earth. However, McGill points to Northern bedrock structures that predate the features said to mark the start of the tectonic process. McGill instead believes that through some novel mechanism the ancient surface sank to its current depth as a single unit. This would explain why features around the basin's edge, which would have formed as the surface dropped, seem to be younger than structures at its floor. The third possibility is that the northern lowlands result from impacts. Some researchers suggest they formed as a series of big overlapping impact craters. Others arguing that the odds against such a pattern of impacts are large, postulate a single event – the impact of an object bigger than any asteroid the solar system now contains.