Scuba divers are exploring the depths of a volcanic lake in Guatemala to find clues about an ancient sacred island where Mayan pilgrims flocked to worship before it was submerged by rising waters.

Samabaj, the first underwater archaeological ruins excavated in Guatemala, were discovered accidentally 12 years ago by a diver exploring picturesque Lake Atitlan, ringed by Mayan villages and popular with foreign tourists.

"No one believed me, even when I told them all about it. They just said 'he's mad'," said Roberto Samayoa, a businessman and recreational diver who grew up near the lake where his grandmother told him legends of a sunken church.