Baltic Sea Anomaly
On June 19, 2011 the "Ocean X Team" found what has been called "The Baltic Sea Anomaly." It became popular due to the sonar image of the structure looking like an air craft. Since then more tests have been done and it's been determined to be made of natural materials.
It's definitely there but the debate is if it's a man-made structure or not. If it is then it would pre-date the last ice age and that wouldn't fit into the current consensus about civilization's history.
"The object had an egg-shaped hole leading into it from the top, as an opening. On top of the object they also found strange stone circle formations, almost looking like small fireplaces. The stones were covered in something resembling soot,"
"It has these very strange stair formations, and if it's constructed, it must [have been] constructed tens of thousands of years ago before the Ice Age." -Lindberg