In 2006, a huge fossil was discovered — so big that it had to be broken up for transportation. It contained two near-complete dinosaurs, locked in battle — a 10-ton plant-eating triceratops and another formidable, unknown predator, one-tenth the size of the triceratops but deadly enough to apparently fell the armored giant. Its discovery reopens a cold case — and could change what we thought we knew about the prehistoric world.