An area known as the Twin Megaliths at the Yonaguni Monument -Vincent Lou, Wikimedia // CC BY 2.0

In 1986, a diver looking for a good spot to watch hammerhead sharks off the coast of the Ryukyu Islands in Japan came across an extraordinary underwater landscape.

The area reportedly looked like an ancient submerged village, with steps, holes, and triangles seemingly carved into the rocks.

Ever since it was first discovered, controversy has surrounded the site that’s become known as the Yonaguni Monument, with some researchers—such as marine geologist Masaaki Kimura—arguing it is a clearly manmade environment, perhaps a city thousands of years old and sunk in one of the earthquakes that plagues the region.

Others believe it’s a natural geological phenomenon reflecting the stratigraphy (layers) of sandstone in an area with tectonic activity. The area is open to scuba divers, so the really curious can strap on air tanks and decide for themselves.

Source: 6 Strange Maritime Mysteries | Mental Floss