Sea turtles are one of the Earth's most ancient creatures. The seven species which can be found today have been around for 110 million years, considering the time of the dinosaurs. The sea turtle's shell, or "carapace" is streamlined for swimming through the water. Not like the other turtles, sea turtles cannot retract their legs and head into their shells. Depending on the species their color varies between greenish, yellow and black. Green sea turtles can stay under water for as long as five hours even though the length of a feeding dive is usually five minutes or less. Turtles heart rate slows to conserve oxygen: nine minutes may elapse between heartbeats. All turtles breathe air, and must surface at regular intervals to refill their lungs, although many turtles spend large amounts of their lives underwater. They can also spend much or all of their lives on dry land. Coming ashore is very risky for sea turtles - they can easily overheat and die. For that they only nest at night or in cool weather.