Ultimasaurus olympians (Ultimate olympian lizard) was the largest dinosaur that ever walked the Earth. Unlike most of it's contemporaries in scale, Ultimasaurus was not, as it's discoverers in Late Cretaceous Montana thought, a sauropod, but a hadrosaur, essentially Tenontosaurus on steroids.
Ultimasaurus was close to the size of a blue whale, which made it the largest and heaviest land-based animal to ever live. Scientists are still trying to wrap their heads around how it could exist without being crushed by it's own weight, though many agree it was probably amphibious. However, most people think that it's unusually large lung capacity and storage of hydrogen was key to it's immense scale. Ultimasaurus would have had to consume 2,000 tons of food a day, clearing entire forests in under a week. A herd would have been a sight to behold in prehistoric North America, and surely a massive temptation for any predator with lots of claws, teeth and nerve. A full-grown Ultimasaurus would have been a massive, indestructible wall of flesh, but juveniles, around the size of elephants, would have been too tempting a target for a T. rex or Thanatosaurus pack to pass up. One released into the modern world would wreak extreme havoc. But by and large, Ultimasaurus is a gentle giant and it was truly the king of the Late Cretaceous.