It’s rainy and windy as our group lands with hopes of experiencing one of the strangest places on Earth: the Yonaguni monument.

This spectacular rock formation was discovered by divers in 1986, and ever since, its origin has been shrouded in mystery and controversy. Is the site natural or man-made? Perhaps a natural formation modified by humans? Or was it quite simply made by extraterrestrial beings that settled here and made some kind of weird landing site for their otherworldly craft? The theories are many — and a Google search doesn’t necessarily make you any wiser. So I opt for strapping on a tank and seeing it firsthand.

Yonaguni is a tiny, windblown sandstone rock barely jutting out of the Pacific Ocean, pretty much as far away from anything as you can possibly get. We make our way to the island in March — which we’re told is the best time of year to dive the monument. At the dive center is a 3-foot model, made of Styrofoam and painted gray, and employees show us exactly where to go to get the most out of the dive. They also explain what conditions to expect in different places.

Once I hit the water, I’m greeted by an unforgettable sight — the likes of which I don’t think I’ll ever see again. The formation is 450 feet long, 120 feet wide and 95 feet tall. We start though a narrow passageway with a giant boulder on top. Or is it a tunnel? Through the other side, the north end of the monument towers above us, from 35 feet deep almost to the surface. Along a perfectly smooth, angled wall two obelisks are neatly placed next to each other. We swim along another flat wall until we reach a corner at 45 feet and are told to wait. We know why. Around the perfect angle, the current’s ripping; the roaring water almost tears my mask off as I peek.

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Around we go, and the Yonaguni monument reveals itself in all its majestic, mysterious beauty: Ahead of us are several 60- to 100-foot-long terraces or giant steps, so perfectly angular that they could be coming straight out of a DIY gardening catalog. With eyes like dinner plates, we slowly make our way against the current, constantly trying to find a height (or depth, rather) where there’s slightly less resistance. Some of us tumble laughingly back to the starting point at the corner and start this weird horizontal climb along the smooth terraces over again.

It’s like swimming on top of a Maya temple with narrow passageways, dead-straight steps and terraces, angled corners, holes, channels and intricately shaped rocks. There’s no way this is natural! I switch between believing in aliens or an ancient civilization — but then doubts set in.

Could it be natural, after all? Or something in between? The consensus among scientists who have studied the Yonaguni monument is that the rocks are perfectly natural. Some claim they may have been partly modified by humans at some point, and discoveries of a few tools lend credibility to this theory. We were told (but not shown) a carving that resembles ancient Japanese scripture has been found — but no extraterrestrial hieroglyphs, anyway. The Yonaguni monument has been a mystery ever since it was discovered by Kihachiro Aratake, the owner of the Sou-Wes dive center, in 1986.

At the end of the giant steps, we swim onto the top of the monument, still a little lightheaded after the (well, almost) otherworldly experience. The plateau offers yet more bizarre rock formations, astonishing channels, potholes and crevices that seem to have been purposefully carved. Are they watering channels? Old fireplaces? Ancient sacrificial sites? As we approach the end of the monument I feel pretty sure that they at least sacrificed turtles here, because a huge rock turtle is staring me fiercely in the face. Seen from above, the rock is almost star-shaped, and it’s almost impossible to comprehend how nature can produce something so geometrical with perfect angles in several directions. Did people on Yonaguni worship turtles in ancient times? Do aliens find them tasty? Or is it just the natural tendency of sandstone to break into angles we’re observing? I change my mind several times during the dive. It’s just weird.

Back on dry land we go for a drive along the coast. We see many examples of huge boulders having broken loose, and some of them had fallen into the ocean. The explanation is that Yonaguni is not volcanic, even though it looks to be at first glance. The sandstone is rich in magnesium, which is a light metal and therefore erodes very quickly and gives the surface a pockmarked, volcanic-looking face. Crevices and caves form when the magnesium disappears, and their eventual collapse creates bays or coves between what is left standing — which forms the points.

Yonaguni lies along the Ring of Fire, an area with a high concentration of earthquakes due to huge faults in the Earth’s crust along the edge of the Pacific. In 1995, parts of Yonaguni were destroyed by an earthquake, and this was surely not the first time it happened. Did earthquakes and erosion possibly play a part in forming the Yonaguni monument? When I consider this, I have no problem accepting the whole thing to be a natural phenomenon — the monument could be a part of the coastline that broke off and fell into the ocean. The angular formations seen today are simply the fracture surface. Maybe. I can’t see another sensible solution, since the sandstone surely would have been eroded and rounded off within a relatively short time underwater. The straight, angular formations simply wouldn’t last long, especially not in the turmoil of waves along the weather-beaten coast.

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If this theory holds water, it means the monument is quite young, possibly just a few hundred years old. But I’m no geologist. Perhaps it was built by little green men to honor the giant turtle believed by the Maya to carry the Earth on its back. The blend of these two theories seems plausible for a second, before I shake it off, pull on my mask and get ready for another dive at what must be one of the most bizarre, strange and mysterious dive sites in the world.

Want to Learn More About Scuba Diving Japan’s Yonaguni Monument? Read Stonehenge of the Deep by Ken McAlpin