Answer: The asteroids which plunged through the atmosphere of the Earth and crashed.
Explanation: Meteor Crater (Barringer Meteorite Crater) was formed 50,000 years ago as a result of an asteroid plunging through the atmosphere of the Earth and then crashed. This occurrence formed into what would become central Arizona.
Meteor Crater is currently the best protected and preserved impact crater on planet Earth this is simply because of the crater's young age and the dry climate it possesses.