The Leatherback Sea Turtle is unlike all other turtles because they are the only ones that have a protective covering over their shells. Their added protection is a leathery skin and a slightly flexible shell which helps them to swim as deep as 4000 feet (1219 m). Citation and more information at https://www.nwf.org/Educational-Resources/Wildlife-Guide/Reptiles/Sea-Turtles/Leatherback-Sea-Turtle