Several years ago I was travelling around Italy and doing a bit of long distance cycling and swimming with a friend. One day we were swimming 2 or 3 miles along the coast of the Cinque Terre in Italy heading north from Riomaggiore, about 3-500m off the coast. The sea was sparking clean and underwater visibility seemed to be 1000m (probably not, but it was amazingly clear).
During the swim we both stopped after observing a series of underwater pyramids. It was honestly the most unexpected thing I've ever seen. The sea bed seemed to be some 20-30m below and was completely sandy without any growth. On it were a series of pyramids perhaps 3-5, but this is something like 8 years ago now. The sides of the pyramids had cut-outs so you could see through, but it was too far down to investigate.
After almost expecting lights to start flashing and then the sea bed to rise up into the sky, nothing happened and we continued our swim. After returning to the hotel I immediately went online to try and find out what these things were. Nothing. I was possibly thinking they might be there to encourage coral growth. Every so often when I remember the experience, I go online and do a search, but still haven't found anything.
From memory I draw a poor (un)artist's impression of what it looked like. Has anyone got any idea?
Just in case anyone thinks I'm making this up, I swear this is genuine.