There's some 200MW capacity (only 80MW is currently used or being built) data center in a mine near a fjord in Norway. The limiting factor for its capacity seems to be the capacity of the electricity net nearby (which you can just hit and the DC is useless). With how NIMBY Norwegians are there's no way it would ever get to a GW or more even if it might be both technically and economically feasible. That's just one project and can't be replicated anywhere else. It's just a massive advantage that above ground datacenters are relatively 'figured out' and similar everywhere.
edit: 61.9340107, 5.5084207 are the coordinates