For more than 150 million years, dinosaurs dominated Earth. They were so successful that other animal groups - mammals included - had little chance of playing anything more than secondary roles.Then, 65 million years ago, the dinosaurs vanished from the world forever. Did they meet a quick and catastrophic end, or did they fade away gradually?In the search for answers to what killed the dinosaurs, scientists have looked beyond fossils. Geological evidence also holds clues and has contributed to many hypotheses, working explanations of how dinosaurs may have become extinct.The extinction mystery is far from a simple "whodunit." The same piece of evidence is sometimes subject to multiple interpretations. And, as yet, there is no obvious "smoking gun," no piece of evidence that strongly supports only one hypothesis while disproving all others. So what do we know about dinosaur extinction, and how do we know it?Scientific evidence and observation are the building blocks of hypotheses. Initially, the same evidence and observations may support different hypotheses. As more evidence becomes available, some hypotheses are substantiated, others are disproved, and new ones are formed.A dinosaur extinction hypothesis is a testable statement describing factors that may have contributed to the dinosaurs' demise and how long the process may have taken. Evidence, observation, and experimentation can serve to support or disprove a hypothesis. Regardless of its ultimate acceptance or rejection, though, a valid hypothesis provides direction for future scientific inquiry.