Impact crater

Impact craters are formed by meteorites or comets striking the Earth or other solid body. There is a system for recording and assessing craters. 
Largest craters on Earth.
The five largest "confirmed" impact structures are:
There are some other suggested impact structures which are larger. It is a feature of the Earth that climate, weathering and plate tectonics removes most of the older features and events. In comparison, the Moon retains a nearly complete record of its past impact events. The implication is that the Earth once suffered a similar bombardment in its early history. This period is known as the Late Heavy Bombardment, because it occurred after the Moon was (presumably) knocked from the Earth.
Identifying impact craters.
The distinctive mark of an impact crater is the presence of rock which has undergone shock-metamorphic effects, shattered or melted rocks, and crystal deformations. Examples: