“Because of the Moon’s lack of water, atmosphere, and tectonic plates, there is little erosion, and craters are found that exceed two billion years in age. The age of large craters is determined by the number of smaller craters contained within it, older craters generally accumulating more small, contained craters.
The smallest craters found have been microscopic in size, found in rocks returned to Earth from the Moon. The largest crater called such is about 290 kilometres (181 mi) across in diameter, located near the lunar South Pole. However, it is believed that many of the lunar maria were formed by giant impacts, with the resulting depression filled by upwelling lava.”
Video Credit: NASA Goddard