Dubbed as one of America's most controversial archaeological discoveries of the 21st Century, Wisconsin's mysterious pyramids are fascinating underwater structures that have an unexplained history.

Early on, stories of underwater stone structures were heard by European settlers and dubbed as old wive's tales from  Native dwellers. However, as more and more people reported seeing stones lining the lake floor during droughts, exploration work was carried out.

After extensive research by teams of divers and scientists who used SONAR technology, at least nine complex structures were found on the lake floor. Some of these structures were rectangular pyramid-like, and others were delta shaped. At another end of the lake, duck hunters reported seeing conical structures which were later confirmed to exist.

While there have been some theories that early inhabitants built the structures as primitive dams to control water flow, some believe they might have served a ritualistic function.

Are these pyramids remnants of some ancient civilization that inhabited the place before the area was covered with water? For now, that remains a mystery.