An impact crater is a large hole in the surface of a planet that is most often caused by a collision with a large space object, such as a comet or an asteroid. All planets and most satellites have impact craters, and even asteroids have impact craters. The Moon is our most obvious example of impacts on a planetary body, as the surface is dotted with hundreds of craters.

To date, scientists have identified over 150 impact craters on Earth. Most have been found on the surface; fewer than a dozen or so are buried or found deep in the oceans. There were probably many more craters, but erosion—from wind, water, or the movement of the continental plates—has erased any evidence of their existence. In addition, there may be many more craters under the thick, vegetative growth of the jungles, in high mountains, or buried deep under sediment on land or in the oceans. The largest known crater, the Vredefort crater in South Africa, is also one of the oldest, with an age of over two billion years. Another large impact crater, the Sudbury in Canada, is a major source of certain metals. Craters on the planet Mars dwarf Earth’s craters. The largest impact crater (also called a basin) on the red planet is Hellas Planitia, measuring 1,243 miles (2,000 kilometers) in diameter.