FYI: You can actually deplete the ground temp over time, but drilling further down helps as does storing excess heat in summer to raise the temp below a bit (yay! free cooling!). There's a ton of research on this and these kinds of systems are very common in Europe. They go very well with floor/ceiling (i.e. radiant-based) heating systems as opposed to the more common air-based heating systems used in the US. It's also _way_ more energy efficient.
Interestingly, mixed over/underground tunnel systems like the London Underground effectively pumps heat back underground (the trains heat up from both the sun and braking etc., and radiate that heat into the tunnels) to the point where the clay surrounding the London Underground tunnels is many degrees hotter now than when the London Underground opened. Cities with underground systems might effectively have a lot of excess heat available to pump out. There's some feasbility study being done on reusing waste heat from ventilation, but I don't know if there's been any work done on to what extent the London Underground heating of the ground in the city centre does to the efficiency of ground source heat pumps.
Yeah, that was my though as well. If every building in Brooklyn tried to do this I'm not sure what would happen to the foundations of the buildings and I think the ground would just converge on some temperature that was just high enough to keep using the system but not that practical either.
