An apatosaurus was a massive plant-eating dinosaur that lived about 150 million years ago during the late Jurassic period. Picture a creature as long as three school buses parked end to end, with an incredibly long neck stretching up to munch leaves from tall trees, and an equally long tail that it might have used like a whip for defense. An apatosaurus could weigh as much as four or five elephants combined.

These gentle giants walked on four thick, column-like legs and spent their days eating enormous amounts of plants. Scientists think they may have swallowed stones to help grind up tough vegetation in their stomachs, since they couldn't chew very well. Their fossils have been found in the western United States, particularly in Colorado, Utah, and Wyoming.

You might have heard this dinosaur called a brontosaurus, which was once thought to be a different animal. For many years, scientists believed the two were actually the same dinosaur and that apatosaurus was the correct name. More recently, some paleontologists have argued that brontosaurus deserves to be considered a separate species after all. Either way, when you see those long-necked dinosaurs in museums or books, you're looking at one of the most impressive creatures that ever walked the Earth.