The so-called Baltic Sea Anomaly is one of those rare cases of hard physical evidence from ancient technologically advanced civilizations that is still around and maybe still (partly) functioning. Discovered recently and covered up by governments who like to keep all that knowledge and technology secret for their own purposes, it is lying at the bottom of the ocean, in the Baltic Sea between Sweden and Finland. However lying at the bottom of the ocean is making it easier for ordinary people to investigate.A Swedish diving team discovered it by sonar in 2011, and dove all the way down to the object to gather data in October of 2012. So far it has been very revealing, but at the same time also very mysterious. It was hardly surprising that the military got involved too. So far the diving team is not going back, partly because of financial reasons, partly because of the military involvement (behind the scenes).
If you want an overview of the many data pieces gathered and the possible identification of the ancient structure, go to my article: The Baltic Sea Anomaly.