There are more than 400 different kinds of sharks. Sharks live in all of Earth's oceans: The largest shark is the whale shark. It can grow to over 12 metres (40 feet) long. That's as almost as long as a school bus. Sharks are older than dinosaurs. The first dinosaurs appeared about 240 million years ago. The oldest known sharks appeared more than 420 million years ago. A shark's sense of smell is so good that it can smell a teaspoon of blood in an Olympic-sized swimming pool. The hammerhead's wide-set eyes let it see better.