The Mexican island of Cozumel is as famous for its marine life as it is for its sandy white beaches. For decades, underwater explorers like Jacques Cousteau have flocked to the tiny outpost to enjoy its colorful coral reefs and a wide assortment of undersea creatures. However, one team of divers encountered a spectacular—and surprising—sight that wasn’t a cluster of tropical fish.

According to The Telegraph, the group had just finished exploring the wreck of the Felipe Xicotencatl, a 184-foot Mexican Navy minesweeper that sunk in 2000 and is now a popular underwater attraction. Suddenly, they heard a noise and spotted a submarine gliding by. The name Atlantis was emblazoned on its side, but no one knows what the vessel was doing or where it was going. In less than a minute, it was gone.

'I've been diving for 10 years and that is the most unique thing I've ever seen,” Troy Knabe, a diver from Eugene, Oregon, who filmed the passing submarine, told The Daily Mail.

To see the eerie footage for yourself, check out the video above.

Banner image courtesy of YouTube.

[h/t The Telegraph]