Divers working off the coast of Rostock, Germany, have found the broken-up remains of a wall on the bottom of the Baltic Sea. Researchers believe the Stone Age construction is over 10,000 years old, potentially making it the oldest human megastructure in Europe, and was likely used to herd prey during hunts.
https://gizmodo.com/stone-age-megastructure-found-at-the-bottom-of-the-balt-1851264285