Which mammal can stay underwater the longest?

sperm whale
   The sperm whale (Physeter macrocephalus) is the champion diver of mammals, staying submerged as long as 2 hours at a depth of up to 6,560 feet. Within the whale's head is a cavity that holds up to 5 tons of sperm oil. As the whale dives deeper, the oil hardens and becomes heavier, helping the animal descend. As the whale surfaces, the oil softens into a liquid, making the whale more buoyant.
   Each day a sperm whale eats 2,000 pounds of giant squid, octopus, and cuttlefish. This diet maintains the whale's body: 60 feet in length and weighing as much as 140,000 pounds (70 tons). The sperm whale was once hunted to near extinction. Currently the world's popula­tion of sperm whales is about 500,000; the number killed by hunters has decreased drastically during the past 40 years.