The amazing video above, showing a chimpanzee and an orangutan swimming and diving underwater, is sure to start people talking about the controversial ‘aquatic ape’ hypothesis again:

For the first time, researchers have documented a behavior that had been thought unnatural, if not impossible: apes swimming.

The scientists captured on video two examples of apes that could swim quite naturally. The first was a young chimpanzee named Cooper, who not only figured out how to swim but could also dive underwater to retrieve items from the bottom of a swimming pool.

“We were extremely surprised when the chimp, Cooper, dived repeatedly into a swimming pool in Missouri and seemed to feel very comfortable,” Renato Bender, a researcher at the University of the Witwatersrand in South Africa, said in a statement. “It was very surprising behavior for an animal that is thought to be very afraid of water.”

The second ape was Suryia, an orangutan living in a zoo in South Carolina. Suryia was recorded swimming about 39 feet (12 meters) without assistance. Both apes use a modified breaststroke to navigate, which sets them apart from other mammals (including humans) that usually dogpaddle when forced to swim.