Gyeonghoeru Pavilion in Gyeongbok Palace was actually a damp marsh.

The western side of Gyeongbok Palace was originally a marshy area where water seeped out when dug. The pavilion built on top of it actually tilted to one side. Pumping out the water was useless because it wasn't rainwater from the sky, but water pushing up from underground. So, instead of removing the water, Joseon gathered and contained it in one spot. They dug out the entire marsh and made it the lowest point within the palace grounds, causing all the surrounding water to flow there, leaving the foundations of other buildings dry. The excavated soil wasn't discarded but became a hill behind the Queen's chambers.