Great things take time.
Qiandao Lake is a man-made lake located in Chun’an County, China, where archeologists have discovered in 2001 ruins of an underwater city. The city is at a depth of 26-40 meters and was named “Lion City”. There would have been 290 000 people living in this city during more than 1300 years.
In 1959 the government decided to flood the area to make a hydroelectric power station. So I wouldn’t say it was discovered in 2001, they just forgot it was there for 53 years.