But to a tourist, it is a different experience altogether, something completely unfamiliar to anything they've experienced before. The London Underground is both London and underground, both complicated and difficult for a foreigner. You may scoff at the exaggeration, but take time to consider the above, what your senses undergo during an subterranean sojourn. Eventually, one gets used to it and is numbed to the experience. But the fact still remains that there exists a whole other world under London, with its own set of customs of where to look and how to act, with its own set of laws enforced by distant men watching through security cameras, with its own physics and environment of unnatural passages and earthy smells and surfaces.
People are afraid to go underground because that is the world their fears inhabit.
The darkness, the unknown, tickles the imagination into believing it saw things which weren't there. The underground is associated through philosophy with ignorance and illusion, through religion with the devilish Dantean bolgias, through physics with scorching magma and brimstone, through literature like Verne's dangerous, eventful journey to the center of the earth. As one progresses inwards, he becomes less man, less rational, more bestial, and so on, as if the surface and sun somehow provided human nature. I, for one, descend into the earth with a light step, because through experiencing the flipside of what is considered reality, by entering worlds like the London Underground, I learn more about what it is actually like to live on the surface. The underground is subject to different laws than the surface, and one cannot appreciate the difference unless he goes through it first-hand.