Raiders of the Lost Toilet Factory

"Isaac Asimov has three laws of robotics, but I've only come up with two laws of draining--and we've already broken one of them," Alex says.

We take the smaller tunnel, which quickly turns into a rectangular concrete hallway about 8 feet high and 4 feet wide. After a stretch of this, we come to a brick-walled room whose concrete roof is partially collapsed, exposing jagged, brittle shards of rusted rebar.

Downstream the tunnel profile narrows again and takes on the familiar shape of a bell. Five minutes of waddling, stooped progress confirms our suspicion that we are back where we started, in our first tunnel. As we head to the exit we marvel at the apparently haphazard layout, and, of course, The Lid.

One of Alex's laws did survive the trip, though--the air when we finally reach the tunnel mouth and daylight is still two days colder than the air inside.

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Urban exploration is an awkward fit in any category: It is a sport, a hobby and a discipline all at once. The skills it demands come from all fields. UE is sometimes called "urban speleology" in recognition of its kinship to caving. Rock climbers also share some of its equipment and techniques; so do burglars. "Buildering," climbing tall buildings, is another close cousin, as is BASE jumping. BASE is an acronym for "Building, Antenna, Span, Earth," and gaining illicit access to such features is part of that sport's underground allure.

At the core of UE, however, are more traditional pursuits. Abandoned buildings are, in a sense, historical documents, and many explorers exhaustively research their sites on the Internet, in libraries and in public archives. Urban exploration also provides almost unlimited opportunity for compelling photography, and some UE Web sites are packed with stunning images. Ruined buildings provide rich contrasts of texture and depth, as well as an air of haunting melancholy.

Photo equipment also serves as a useful "credibility prop," anything designed to allay the authorities' suspicions. "I can't tell you how many people have used the 'photography course' excuse," Vale says. Some explorers use disguises to "social-engineer" their way into challenging sites: suits for office buildings, coveralls for factories. Ninja attire might help explorers avoid getting caught, but it's awfully hard to explain if they are.

"Don't wear all black or anything like that," Vale told me before the factory mission--but then he added, "Don't wear white either."

In other words, an urban exploration mission might require you to read up at the library, don a disguise, foil a motion sensor, climb down an elevator shaft, take a light reading for a photo and concoct a convincing story for a security guard. For a certain kind of person, it can be the perfect adventure. Such exploits, however, aren't exactly commonplace. More often, expeditions consist of explorers quietly wandering a site--if they get in at all.

"We're probably about 80 percent unsuccessful," Dirtbag says. "But even when you fail, it's just driving around with your buddies."

Dirtbag, a 28-year-old Dallas resident, is tall, rangy and clean-cut. He habitually wears a placid expression and speaks economically, leaving Vale to dominate the conversation. Vale, 24, sports a dark beard and shaggy hair, and his habit of thinking aloud reveals the mind of a philosopher. The two met in college in Corpus Christi, and though Vale now lives in Amarillo, he often comes to Dallas to explore, lured by the continuous cycle of construction and abandonment.

"Dallas is a really metabolic city," Vale explains. "It's constantly digesting itself."

Dirtbag agrees, calling Dallas "the smorgasbord of UE," but in their quest for new sites the two have explored much of Texas. They document their peregrinations on their Web site, www.texploration.org. Vale has gone even farther afield, including Toronto last year for an urban exploration convention. More than 60 explorers met to compare notes on favorite sites and techniques. Characteristically, the convention had its own fake name: OPEX, short for Office Products Expo 2004.

In a sense, even the name "urban exploration" is misleading, because many factories, airports and abandoned military bases are found in rural areas. Nevertheless, for sheer variety, the supreme American UE playground is New York City.

In 1993 a journalist named Jennifer Toth wrote The Mole People, a book that revealed the society that thousands of homeless tunnel dwellers had constructed beneath Manhattan. Though critics said Toth's account was exaggerated, some of its substance was confirmed, and interest in New York's tunnels soared. New York exploration gurus the Jinx Society added to the genre with Invisible Frontier, and last year the grande dame of subterranean New York, Julia Solis, came out with her much-anticipated New York Underground.

Interest in UE may be stronger outside the United States. Many consider the legendary Paris catacombs as the discipline's birthplace, while some of the earliest known UE groups were formed in Australia. In terms of Web presence, the Canadian UE scene dominates its southern neighbor. Canada boasts both UE's definitive forum, Urban Exploration Resource (www.uer.ca), and its best known e-zine, Infiltration.org.

Since its founding in 2002, UER has mushroomed into a community with more than 6,000 registered users. Yet it is impossible to tell whether the growth of urban exploration's Internet presence is fueled by increasing interest in the hobby or by existing explorers simply finding each other on the Web. In the forum, the enthusiasm in the first-time posts from explorers who thought they were alone in their odd obsession is almost touching. Vale describes his feelings when he discovered the online community two years ago: "It was like, 'You mean I'm not the only weirdo out there?'"