Evidence found in Denver holds the key to unlocking an ancient mystery about when humans first arrived in North America.
For millions of years, New York has been the centre of the action. Mammoths once trampled down Fifth Avenue and dinosaurs roamed where the Giants now play.
Chicago invented the skyscraper, but it has a history of hosting towering beasts. 13,000 years ago, it was home to creatures whose bones are still found in the city's parks.
Thousands of years ago, Washington D.C. was patrolled by dinosaurs, and flying reptiles terrorised the skies over what would later become the National Mall.
Denver was once home to the T. Rex and Triceratops. Evidence also suggests it holds the key to unlocking an ancient mystery about when humans first arrived in North America.
Travel back over millions of years to when oversized armadillos, scimitar-toothed cats and the largest ocean-going reptile ever to exist - the Tylosaurus - roamed Dallas.
Long-necked sea creatures fought underwater battles, continental plates clashed, earthquakes rocked, and volcanoes spewed - all in Los Angeles before it was America.