The quest for exceptional fossils drives paleontologists to remote, poorly explored places. Every summer since 1990, American Museum of Natural History scientists Mike Novacek and Mark Norell have trekked halfway around the world to lead expeditions in the Gobi Desert in Mongolia. Mike's specialty is mammals that lived millions of years ago. He wants to understand which prehistoric fossils are related to modern mammals — including humans. Mark's passion is figuring out how certain dinosaurs evolved into modern birds. The fossils that hold the answers to these paleontologists' questions are incredibly rare — so they'll go to great lengths to find them.