Having legs located far back on their body makes it necessary for penguins to walk upright when on land. While some of the smaller species are fairly coordinated, the larger ones (such as the Emperor penguin) are clumsy on land. Penguins do not always have to walk, however. On steep icy slopes, they may travel by sliding on their belly, using their feet to steer and the flippers to steady themselves.
Underwater, penguins can move swiftly; their normal speed is 3-6 mph (4.8-9.6 km/h), although they can move faster for short bursts. They have three basic modes of transporting themselves in the sea. The first form is underwater flight, used for rapid movement when feeding or avoiding a predator, such as a leopard seal. The second form is known as porpoising, and is used for traveling longer distances. When porpoising, penguins alternate between swimming deep in the water and leaping out of the surface. It is thought that this increases the overall speed by reducing water resistance when swimming; also, this type of swimming allows the bird to catch a quick breath of air without stopping. The final form of swimming is the duck-style, with head and tail held erect, which penguins assume just before going ashore, in order to orient itself.
Penguins occasionally dive deeply in their quest for food, which consists primarily of fish and crustaceans. For example, Emperor penguins reportedly can dive 850 ft (260 m) below the surface and remain there for about 18 minutes.