[MeanPi2]

Most of what Mark said. BUT...

If you're simply talking about the action INSIDE the Umbrella structure underground, the illusion of 'underground' was simply an illusion, in the movie they told people it was underground with graphics in a scene on a laptop but within the underground itself, in the movie, there was no proof except 'outside' wasn't visable (except one scene where the reintereated what was there wasn't real) and in actuality everything is a dead-end - even if there were apparent entry and exit points, but none of them were really there. On the DVD narrative (I love this movie for some reason) they only really had 1 door, or maybe 2 but no more than that and those structures where the breast broke out of, they only had a couple of those and had to move them around for each shot. In essense each 'underground' shot was a stage, could be put inside it's own little box, built there, filmed, then a new box is created for the next scene, built and filmed and so on.. 'Underground' is simply visual isolation from the earths surface by telling people that beforehand (in the cheapest way by, for example, having the words: "Earth, 2104, after 2 years of nuclear warfare the Earths surface has become a barren waste uninhabitable by even cockroaches and humanity, in humanities genetic need to survive, have gone underground.." subtitled on the opening frames).