Sea turtles are among the largest living reptiles. They have scales and a bony shell, are cold-blooded, breathe air, and lay their eggs on land. Sea turtles are long-lived, although scientists are uncertain how long they live because there is no known way to determine their age. Unlike the land turtles from which they evolved, sea turtles spend almost their entire lives in the sea. They glide gracefully through the water with flipper-like forelimbs and a streamlined shell. Sea turtles frequently come to the surface to breathe when active, but they can remain underwater for several hours when resting.