Fascinating history summary by physicist John Baez:
During the Late Heavy Bombardment, the Moon was hit by 1700 meteors that made craters over 100 kilometers across. The Earth could easily have received 10 times as many impacts of this size, with some being much larger. To get a sense of the intensity of this pummeling, recall the meteor impact that may have killed off the dinosaurs at the end of the Cretaceous period 65 million years ago. This left a crater 180 kilometers across. Impacts of this size would have been routine during the Late Heavy Bombardment.