Thirty years ago, divers were looking for a good place to look for hammerhead sharks off the coast of Japan. But they came across something scarier than sharks and quite strange - giant steps under the sea.
Now known as the Yonaguni Monument, no one can agree on whether they're entirely natural, been modified by man, or made by man.
It does look like it's manmade - it's a mixture of mudstone and sandstone and it stands off the ground - it's huge at 490ft by 130ft and around 90ft high.
Masaaki Kimura, a Professor of Science at the University of the Rykukyus in Okinawa is adamant that it's manmade. He says that the formations are monoliths created by an ancient civilisation.
On the actual island of Yonaguni, Kimura notes that there are small camps, pottery, stone tools, and large fireplaces that date back to 2500 BCE. But that doesn't mean they had the knowledge to create monoliths.
But geologist Robert Schoch thinks it's entirely natural. If it were manmade, he argues, it would have to have been built when it was above the sea - and that would have been over 8,000 years ago before the Ice Age. It's simply not that old.
Secondly, the Yonaguni monument is made out of "solid living bedrock," according to Robert. He says: "No part of the monument is constructed of separate blocks of rock that have been placed into position."
"At at the surface, natural wave and tidal action is responsible for eroding and removing the sandstones in such a way that very regular step-like and terrace-like structures remain," he concluded.
But no one wants to believe him!
Some people think it's evidence of the lost continent of Mu. Amateur archaeologist Augustus Le Plongeon (great name) thought that Egyptians and Native Americans were actually refugees from Mu, a lost continent that he thought was bang smack in the middle of the Atlantic Ocean.
Not a huge amount of people believed him at the time, to be honest.
I can see why he thought there should be some land in the Atlantic - it is a huge body of water but look at it! Doesn't it look ridiculous!
Credit: My Mu
Other people think it must be aliens. Underwater aliens. This is not really a coherent argument as such - it's just that the Yonaguni Monument is a bit eerie to dive round and alien in that it's creepy and otherworldly. And when we can't quite explain something... aliens did it. 100 percent.
The Lost Continent of Mauritia
Speaking about lost continents, they've found another one! How many continents have we lost?! And are we sure they're not just big islands?
Deep under the Indian Ocean, scientists have just found a submerged micro-continent underneath the African island of Mauritius.
Credit: University of Witwatersrand
When it was above water, it would have been almost a thousand miles long and once linked India and Madagascar.
Found underneath the island, the micro-continent has been named Mauritia by amazed scientists.
Of course, now we have seven continents, but Earth's tectonic plates are constantly moving and over millions of years have shifted dramatically. Back when the land was still forming and reforming itself, there was one super continent called Pangea that broke up to form smaller continents - and Mauritia sank, basically. A bit like Atlantis.
Words by Laura Hamilton
Featured Image Credit: YouTube/How Stuff Works