According to Guinness World Records, the emperor penguin is the holder of the deepest dive ever measured accurately. In 1990, an Aptenodytes forsteri reached a depth of 483 m (1,584 feet) in the Antarctican Ross Sea.
The longest lasting bird dive is also in the hands of this super-athlete. An Emperor stayed under water for 18 minutes at Cape Crozier (again, Antarctica) in 1969.