The very rich, having fundamentally missed the point of urban living, have long been frustrated by the fact that it's impossible to squeeze the amenities of a country mansion--car showroom, swimming pool, cinema, servants quarters etc.--into the floor space of your average London terrace. Those without access to trans-dimensional engineering, a key Time Lord discovery, have had to resort to extending their houses into the ground. Thus proving that all that stands between your average rich person and a career in Bond villainy is access to an extinct volcano.