Erosion and plate tectonics have destroyed most of Earth's craters. Fortunately, many ancient impacts left behind distinctive changes in the rocks lying deep beneath the crater. Like the scars from old wounds, these imprints have remained long after the surface structures were eroded away.

These signs of collision, and the few actual craters that still exist, give us our closest look at the impact process and shed light on how collisions have shaped our own and other worlds.

• Learning From Meteor Crater
• Collisions Large and Small
• What Killed the Dinosaurs?

This elliptical impact site in Sudbury, Ontario, is 1.9 billion years old. Originally circular and much larger, the deformed structure we see today consists of rocks that were deeply buried at the time of collision. Mountain-building deformed the structure. Later uplifting and erosion exposed these rocks.