PARIS—While many Parisians go out on the town on Saturday evenings, a small but growing number go under it.

Beneath Paris lies a network of some 155 miles of tunnels known as "the catacombs"—an underground labyrinth that serves as the weekend playground for bands of urban explorers. One recent Saturday, several dozen "cataphiles," as these explorers are known, climbed down an embankment in south Paris to a unused railroad track. After a short walk, they disappear into a hole in the side of a railway tunnel to the catacombs, 65 feet below.

"The environment never changes down here," says Riff, 44, a catacombs veteran of 22 years who won't give his full name because he likes to explore areas off-limits to the public. "Many people come here, I think, because it gives them a milieu in which they can always know what will happen. It's constant."

Sixty-six feet under the City of Light's scenic streets, people are gathering each weekend to explore Paris's true underground arts scene. The 160 miles of centuries-old tunnels have become a unique gallery for artists.

The catacombs started as underground quarries to extract limestone for construction in the 12th century, when the city covered just about a square mile. But by the late 1700s, Paris had expanded over the quarries, triggering a series of catastrophic land collapses that killed dozens.

In response, the government built an array of underground walls to prop up the land surface. Most reinforcing walls were constructed under the street-side of buildings, meaning that in most areas, the underground passages mirror the Parisian street patterns at the time of reinforcement. Workers engraved the names of the streets above ground on the passages below, says historian Gilles Thomas.

"It's like Paris in the 17th century," says Bacchus, a 46-year-old cataphile. "This is a chance to know its underground carbon copy."

Some of the cavities were used to hold human bones from Paris' overflowing graveyards. They were named the catacombs in a nod to ancient Rome's underground cemeteries.

The underground sections with the bone remains can be visited via a museum, which guides visitors through well-lit passages. But the nighttime revelers—said to include people from executives to students and artists—prefer to explore the off-limits areas. There, corridors are about a half-meter wide, sometimes too low for an adult to stand up, and sometimes knee-deep with water. Punctuating the tunnels are caves and rooms that some cataphiles have decked out with stone benches for their purposes—partying or cultural pursuits.

In the Paris Catacombs

Gilles Cyprès 35, paints murals in the catacombs. Recently, he unveiled his third such mural, to 20 friends who popped champagne and munched on crêpes as they stared at a six-foot tall, phantasmagoric dream scene with wide rural vistas and folkloric creatures.

The painting took over a year to finish, he says. "There are lots of complications and technical difficulties. For me it is a challenge and if I am capable of doing this down here then I am capable of drawing or painting anything," he adds.

The police, who are relatively tolerant of the cataphiles, don't block entrances used to access the tunnels; in the past this has led to manhole covers being opened in the street, endangering the public. But the police do fine people up to about €30 ($40) when they catch them below the surface.

"We have met boys in flip-flops, girls in heels and miniskirts," says commandant brigadier Sylvie Gautron, 39, of the Compagnie Spécialisée d'Intervention, the police unit charged with monitoring the catacombs.

She said its number of rescue missions has trebled in the past two years. "They have often a tiny light that might last three hours. Most times, we find people lost in complete darkness."

Maps used to be guarded by catacomb initiates, and were hard to come by for aspiring cataphiles, but advances in technology helped the distribution of blueprints. The first photocopied maps began to circulate in the 1980s. In the past 10 years, the Internet has created an explosion in casual visitors—or "tourists," as the old-school cataphiles call them.

"It's been a month that I've been coming here," says Vincent Delate, 14, who downloaded a map from a catacombs website and found his way to the "Castle Room" with a friend. "I know it already by heart," he said, sitting in the candlelight.

The catacombs feature reminders of historic events that unfolded above ground. The secular republican fervor of the French Revolution led, in 1793, to the banning from all public signs of the fleur de lis, the symbol of the toppled French monarchy. But it can still be seen on some signs in the catacombs, though scratched out by hand.

The catacombs include the better-known columns made up of human skulls and bones. Agence France-Presse/Getty Images

The ambitious urban planning projects of Baron Haussmann in the late 19th century saw many small streets swallowed up by expansive grand boulevards. Not so in the catacombs, where passages that have long disappeared above, like Impasse de Longue Avoine, still exist below.

Another catacomb subculture is cartography. The City of Paris has developed its own official map for use by police and other government agencies, but cataphiles have produced their own versions, usually out of pure necessity.

"Nexus," a cataphile who goes by a pseudonym, is the most prominent catacombs cartographer. He has produced dozens of highly detailed maps—an exercise that blends the official naming of places by the state with the vernacular terms developed by generations of cataphiles.

"It's not just about showing the way," he says of his mapmaking. "It's also about putting together the bits and pieces of [catacombs] history."

Write to Don Duncan at wsje.weekend@wsj.com