This is too weird. I'm 94% sure there's some type of intelligence behind all those straight lines and I can't imagine any event that would cause this much stone to submerge to depths of 25 meters or more, leading me to arrive at a 92% certainty that it's many, many, many thousands of years old. 10,000 plus. http://www.freediverhd.com/2014/04/yonaguni.html https://www.youtube.com/watch?v=_ep9P6uX9BM
It would seem that man may have been around here for more than thousands of years, perhaps millions of years, and I'm not talking about any missing link. Did you know that perfect human footprints have been found along side of dinosaur footprints? Could it be that dinosaurs were around much less than 65 million years ago? Look around here some (for starters): Forbidden Archeology (search) https://search.yahoo.com/search;_ylt=AqzHiqwMM18F3WWnc1yLj9ubvZx4?fr=yfp-t-598-s&toggle=1&fp=1&cop=mss&ei=UTF-8&p=forbidden%20archeology
What if they dug up old bones and stamped them into the mud and then stood next to it just to make you think they were around only a short time ago.
Mankind is much, much older than we think. There have been civilizations that predate Sumer and Babylon, which were far more technologically advanced than what we see today.
Maybe it's from a group of your avatar's namesake op. Though usually not considered to operate as efficiently as Modern Humans, Neanderthals are reported to have larger brains than modern humans, so it's not unreasonable to think perhaps they made some interesting structures.