A thousand pound predator swims up to a National Geographic photographer in Antarctic water, opens its jaws, and gently offers him a dead penguin. It keeps doing so for four days. This episode takes that encounter as the starting point for a portrait of the leopard seal, an animal first described in 1820 from a faded, stretched taxidermy skin. The living seal is a countershaded giant up to 11 and a half feet long, with females as much as 50% larger than males, and it powers through the water with its front flippers in the manner of a sea lion.

Its feeding habits seem to belong to two different animals. With inch long canines but no slicing teeth, it tears penguins apart by whipping them against the water’s surface. With interlocking molars, it strains krill from the sea like a baleen whale. Males hang upside down beneath the ice and sing at up to 177 decibels. The episode also weighs the species’ unpredictable record with humans, from Shackleton’s expedition to a fatal attack in 2003, and asks what becomes of an ice loving hunter as the pack ice melts.

• To dive past 80 meters without the bends, the seal collapses its own lungs. Surfactants keep the air sacs from sticking so they reopen at the surface.
• Researchers count leopard seals with underwater hydrophones, because mature males sing a consistent repertoire of five distinct calls.
• Pups weigh 66 pounds at birth and are nursed for about a month before being left on the ice. Roughly 25% die in the first year.
• A scarred female known as Owha lived around New Zealand’s North Island for a decade, and seals there have learned to hunt ghost sharks and stingrays.
• Marine biologist Kirsty Brown was dragged to 70 meters and drowned while snorkeling near a UK research station, the only recorded human death attributed to a leopard seal.