Paris, France, 2:45 A.M.: four persons in dark fatigues slip undetected into the Paris catacombs.

Toronto, Canada, 3:30 A.M.: A man in a ninja mask is questioned by security guards for trespassing in the basement of Toronto's 'Royal York Hotel'.

The night belongs to the urban explorers, a worldwide movement of hardcore adventurers, and thanks to web-sites like 'Planetjinx' and 'Infiltration', their numbers are skyrocketing.

Urban exploration is dangerous and illegal. It's a compulsion that drives young men and women out of the warm safety of their homes, down into the uncharted wilderness beneath the city streets. The very places most despised by decent people, storm drains, rooftops, and sub-basements, are irresistible to urban explorers. They move easily through safety barriers and security perimeters, heedless of warning signs, alarms and armed guards. They're as much at ease smoking a cigar in your hotel suite as climbing the walls of a ventilation shaft. There's scarcely a university campus in North America that doesn't have its own crew, exploring the secret tunnels and off-limits rooftops.

Motivations for urban exploration vary from one group to another. The 'Jinx Project', publishers of 'Jinx' magazine, claim to be engaged in a protracted form of 'psychological warfare' against its perceived enemies.

"We're gathering intelligence, doing reconnaissance, and promoting the cause of freedom," says the project's director. 'Infiltration', a reluctant leader of the worldwide trend, calls the activity "a sort of interior tourism which allows the curious-minded to discover behind-the-scenes sights."

What ancient impetus burns in the hearts of these adventurers? Where has the healthy process of socialization failed them? Civilization is a very recent imposition on our species; only for the past 5% of our man's existence have we lived in houses instead of caves. Today a small but growing minority of our youth is backsliding into atavism, hunting and gathering through industrial wastelands.

Like many a fringe element, they have found a home on the Internet. In addition to urban exploration portals like 'Jinx' and 'Infiltration', there are thousands of sites dedicated to the movement.