Forget Berlin. The Ruhr area, biggest continuous settlement in Germany, with roughly 20 million inhabitants (of which maybe 5 million would be affected), is below its local water table because "the dwarves delved too greedily and too deep". So now they are spending 100Mio per year on pumping away the ground water, probably forever:

https://www.worldtimes-online.com/imfocus/105-pumpen-im-ruhr...

https://www.amusingplanet.com/2021/03/the-pumps-that-keep-ge...