One of the most mysterious dive sites out there, the Yonaguni Monument still puzzles researchers today. The site consists of a submerged rock formation off the coast of Yonaguni in the Ryukyu Islands, Japan.

The sunken monument was discovered in 1987 by a local diver exploring off the coast of Japan’s southern Ryukyu Islands. The structure is a massive 50-meter-long-by-20-meter-wide behemoth and was nicknamed “Japan’s Atlantis.” Some think it is more than 10,000 years old.

No one really knows how the strange stone structures got there. Some scientists believe it is formed geologically, while others claim it may be a remnant of the mythical lost continent of Mu. The largest structure looks like a stepped pyramid that rises from a depth of 25 meters/82 feet, part of the reason why some researchers believe they could be manmade steps or terraces.

Thanks to the myths surrounding it, the Yonaguni Monument has become a relatively popular diving attraction despite strong currents.