A large slab of granite has been found deep in the Atlantic Ocean, Japanese researchers said on Tuesday (May 7), suggesting a continent may have once existed off the coast of Brazil.

A team led by the Japan Agency For Marine-Earth Science And Technology (JAMSTEC) found the 10-metre high by 10-metre wide rock 'cliff' more than 900 metres (2,950 feet) under the ocean while conducting a survey with its deep-sea submersible in the Rio Grande Rise, part of the southwest Atlantic basin some 1,200 kilometres (745 miles) southeast of Rio de Janeiro.

Granite is usually only found on land, and researchers say its presence on the seabed may be evidence of a continental land-mass that was swallowed by the waves at some point in the distant past. A large volume of quartz sand, which is also not formed at sea, was found around the granite slab.

The team has not removed any samples, a JAMSTEC official said, adding that further investigations will be handled by other parties including the agency's Brazilian counterpart.

JAMSTEC is currently searching seas across the globe as part of a year-long mission to look for life in some of the ocean's deepest locations.

In about 360 BC, the Greek thinker Plato wrote that an ancient warlike civilisation -- Atlantis -- had once existed somewhere in the Atlantic, before sinking into the sea in a single night and day.

But academics and writers have argued its real submerged location may be anywhere from the Mediterranean to Antarctica -- with others suggesting the tale was merely a myth.

Presented by Adam Justice