At MIT the exploration of steam tunnels is known as hacking or vadding. The term 'Urban Exploration' first came into wide spread use through Wes Modes, editor of one of the first Urban Exploration web-sites. Another name for the hobby is infiltration, which is also the name of "the zine about going places you're not supposed to go". Note however that the term "urban exploration" generally refers to the exploration of abandoned, subterranean or otherwise unwelcoming environments, whereas "infiltration" more often is used to describe exploration of active buildings via stealth or disguise.
Urban explorers usually are trespassing by visiting the places they do, making the activity a criminal offence. Most explorers justify this by following a code of ethics that has been loosely defined and adopted by the community. The wording and language varies amongst different renderings of this code, but it usually frowns heavily upon theft, vandalism, tagging, graffitti, and any other crime except for simple harmless trespassing. A simpler way to generalize this ethical code is the Sierra Club's motto: "take nothing but pictures, leave nothing but footprints."
See also: Caving, Catacombs of Paris