A substantial part of the problem is the sheer age of central London; people have been digging beneath it and piling more buildings on top of it for nearly 2000

A substantial part of the problem is the sheer age of central London; people have been digging beneath it and piling more buildings on top of it for nearly 2000 years, so given any particular spot in the Tube network, there's every chance that if you try and drill upward from it, you'll hit part of the sewer system, a buried river, someone's wine cellar, something top-secret belonging to the state, a lost graveyard, a plague-pit...