Penguins are true flightless birds. Some species spend as much as 75% of their lives at sea, yet they all breed on land or sea-ice attached to land. To withstand the harsh conditions of the Antarctic, their bodies are insulated by a thick layer of blubber and a dense network of waterproof plumage. Penguins' bones are solid and heavy, which help them to remain submerged and reduce the energy needed for pursuit diving. Some species can reach depths of 1000 feet or more and stay submerged for up to 25 minutes, though most prefer shorter, shallower dives.