About 50,000 years ago, a rock fragment broke away from the asteroid belt and hurtled towards earth. The rock, composed of nickel and iron, was about 50 meters across and weighed 300,000 tons. It was travelling at 12.8 kilometers per second. Upon entering the earth’s atmosphere it became a giant fireball that streaked across the North American sky. When it crashed into the plains of Arizona, it exploded with a force equal to 10 megatons or about 150 times the force of the atomic bomb that destroyed Hiroshima.

The violence of the impact vaporized the meteorite leaving little residue, but millions of tons of limestone and sandstone were blasted out covering the ground for a mile in every direction. When the dust settled, what remained was a crater over a kilometer across and 750 feet deep. The impact occurred during the last ice age, a time when the Arizona landscape was cooler and wetter. The area was an open grassland dotted with woodlands inhabited by woolly mammoths and giant ground sloths. The force of the impact leveled the forest for miles around, hurling the mammoths across the plain and killing or severely injuring any animals unfortunate enough to be nearby.

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