Whales are the largest species of exclusively aquatic mammals, members of the order Cetacea, which also includes dolphins and porpoises. They include the largest known animals ever evolved. Whales, along with most dolphins and porpoises, are descendants of land-living mammals. Like all mammals, whales breathe air into lungs, are warm-blooded, breast-feed their young. Beneath the skin lies a layer of fat, the blubber. It serves as an energy reservoir and also as insulation. Whales have a four-chambered heart. The neck vertebrae are fused in most whales, which provides stability during swimming at the expense of flexibility. Whales breathe through blowholes, located on the top of the head so the animal can remain submerged. Whales have a unique respiratory system that lets them stay underwater for long periods of time without taking in oxygen. Some whales, such as the Sperm Whale, can stay underwater for up to two hours holding a single breath. The Blue Whale is the largest known animal that has ever lived, at up to ninety-three feet long and 180 tons. This series of "Marine Life Protection" proof one dollar coins has been a hit with collectors since the first coin was issued in 1992. It is hard to say whether the obverse or reverse is the most attractive, but they are becoming exceedingly hard to find. Palau was chosen as the location for the 10th season of the hit reality series Survivor, and this coin depicts regional marine life, as well as a beautiful mermaid. This One Dollar coin is legal tender and has been issued by authority of the government of Palau.