Ancient Aliens Episode 9 asks whether the ruins of a city at the bottom of a lake in China point to aliens having visited the country in the distant past.

In 1992 a diver found the remains of a city under the waters of Fuxian Lake in China. Over the course of 35 dives and with the use of a submersible he found dressed flagstone, stairs, walls and other evidence of a large settlement.

Fuxian Lake is in the Yunnan region of China

Finally in 2001, archeologists from a nearby university used sonar and underwater cameras to survey the lake. They found a settlement covering over a square mile with remains of what appeared to be an arena and pyramid.

The lake itself, at 509 feet, is the second deepest freshwater lake in China and covers over 100 square miles.

Who built the mysterious city beneath the waters of Fuxian Lake

Commentators say the location of the city must mean it was built before the last Ice Age and is therefore far in advance of anything else built at the time…many of them point to alien involvement.

Source: Monsters and Critics

Other references: Ancient Pages, GoKunming, Beyond Science