Ten years ago, the Paris police discovered an outlaw movie theater, complete with bar and restaurant, hidden in a sealed-off old quarry tunnel under the 16th Arrondissement, just across the Seine from the Eiffel Tower.

“The whole thing ran off a professionally installed electricity system and there were at least three phone lines down there,” police were reported to say.

Three days later, when they came back to dismantle the illegal theater, it had all been packed up and shipped out, leaving only a single note for the police: “Do not try to find us.”

Every city has an underground.

But in Paris, the underground is literal. Ten percent  of the city is built above underground caves, tunnels and medieval limestone quarries. When building foundations are constructed, the tunnels have to be taken into account, and since no one in Paris has a complete and accurate map of the tunnels, it sometimes becomes a problem.

It is estimated that there are some 180 miles  of tunnel beneath the streets of Paris – and that’s not counting the 1,300 miles of sewers or 125 miles  of subways.

Access to most of the tunnels has been illegal for civilians since 1955, but hordes of young people have been using them for recreation – of various sorts – since they were constructed. Even now, everything from rock concerts to picnics are held in the limestone tunnels by those called “cataphiles,” after the catacombs of the city. Less innocently, they are also the site of many drug deals and other underworld activity.

During World War II, the tunnels provided shelter for the Resistance. In the 1950s, there were underground jazz clubs. A fascination with the underworld is hardly a new thing.

From Ulysses to Dante, we have been deeply curious about a world that seems mysterious and dark, dangerous and illicit. And even for some tourists, a visit to the sunless portions of a city is an irresistible draw.

Luckily, there are legal ways to visit the subterranean Paris.

CATACOMBS

When Paris was young and the Romans gave the orders, many of its important buildings were constructed of limestone quarried in the local hills. As the city grew, the quarries dug deeper into the hills, and the beige limestone city overtook its suburbs and their quarries, which now honeycomb the hills of Montparnasse and Montmartre.

By 1786,  Paris had grown to more than a half-million people, and there was need for fresh real estate. The city council decided it could empty out the cemeteries and use the land to build on, and so, from 1786 to 1860, graveyards were dug up and their bones transported to the tunnels under Montparnasse. In all, more than 5 million bodies made the move.

They are an eerie sight in the dark. All that death, all those bones, all that underworld, Avernus, Chiron, Cerberus.

The trip through the catacombs is long and arduous, with stairs going down six stories into the bedrock of Paris, then a good half mile of twisting narrow tunnels before you ever even get to the bones.

“Stop!” said the sign above the final door. “You are entering the empire of the dead.”

The tunnel contorts around in the underworld, with niches to right and left piled with the bones torn up from Paris graveyards.

Millions of the dead from Paris’s past are piled here, with a wall of tibias and skulls making a kind of bone-dike, holding the remaining body parts behind it. Usually, the bones are assembled almost like masonry, with the knob-ends of the tibia bones left end out, and a line of skulls across them, like strata in rocks, or ornament in brickwork.

The tunnels are never higher than about 6-feet and maybe 2 inches high, and only one person wide. Their floors are often damp or wet with cave-drizzle.

Electric lights are placed on the walls every 10 or 15 feet, but with fairly low wattage bulbs.

Signs were left with the bones describing what cemetery they were disinterred from, almost like regimental monuments in a battlefield.

After snaking through the underworld for perhaps a mile, you came to an even narrower spiral stone staircase with 84 steps, as the sign says, bringing you those six stories up from Hades, and you open out into a side street from a nondescript stone building-front with no markings to warn anyone this is the exit from Tartarus.

The trip is a genuine experience, an encounter with history.

Going through the tunnels is a completely alimentary experience. The bowels of the earth. Expelled into sunlight.

THE SEWERS

The sewer system of Paris is one of the world’s wonders. If straightened out, they could run from Paris to Istanbul.

They have a place in fable and lore far exceeding that of any other sewer system in the world: They are the tunnels through which Jean Valjean carried the wounded Marius in Les Miserables.

“By degrees, we will admit, a certain horror seized upon him. The gloom which enveloped him penetrated his spirit. He walked in an enigma. … How was he to get out? Would he find an exit? Would he find it in time? Would that colossal subterranean sponge with its stone cavities, allow itself to be penetrated and pierced? …Would they end by both getting lost, and by furnishing two skeletons in a nook of that night? He did not know. He put all these questions to himself without replying to them. The intestines of Paris form a precipice. Like the prophet, he was in the belly of the monster.”

The “intestines,” is the right word. They are called in French, Des Egouts — the “guts.”

The entrance to the portion open to the public is like a tiny lemonade stand beside the bridge over the Seine. You buy tickets and descend into the depths, where a very faint odor of sewage wafts up — so faint that in a few moments, you no longer notice it.

What first hits you in the darkness of the sewer tunnels is the sound, the thunderous sound of water, like a torrent over a waterfall. Part of this is the sound amplification of the tunnels, but most is the enormous quantity of water passing through the system. As you walk down one wide tunnel – more like an extended garage — the water passes underneath you as you walk over a steel mesh walkway.

There are signs along the way explaining not only the sewers, but the history of the sewers, beginning in Roman times. The current system is a gift of Baron Haussmann from his city rebuilding plan of the mid 1800s.

There is a kind of pulse, or bloodflow, that the system implies, a circulation system for the city.

One looks at a city from above ground and sees the traffic, the shops, the restaurants, the pedestrians, the apartments above the stores, the metro stations, the statuary and monuments, but they are all just the skin of the city.

Under it run the subways, the catacombs, the sewers, the remains of hundreds of years of quarrying, the abandoned tunnels of previous water and sewer lines, abandoned subway tunnels, and millions of miles of cable.

And, of course, the people milling around on the surface, drinking their cafe cremes, could not exist in the city without the subsurface infrastructure.

THE CEMETERIES

It says something about Charles Baudelaire that he is buried in a grave listed under his despised step-father’s name. It is odd, that someone as famous and accomplished as the poet was, of course, someone’s son, and part of a family, and all the wretched dynamics that implies: Charles was Petit Charles  to his mom and step-dad, and even after he grew up, he seemed to remain their little boy, so that, unlike other more pompous and be-medalled Frenchmen, with their giant sepulture, with statues and trumpeting angels, he gets second billing on a plain headstone in the cimetiere Montparnasse, with his mother — who outlived him by two or three years — billed third.

It is her revenge on her wayward child.

There are three large cemeteries in Paris, and they each have their celebrity tombs.

Most famous is the grave of Jim Morrison of The Doors at the cimetiere du Pere-Lachaise near Montmartre.

There, you can also discover Apollinaire, Balzac, Beaumarchais, Sarah Bernhardt and Frederic Chopin.

At the cimetiere de Passy you find Debussy, Manet and Joan of Arc.

Visiting the cimetiere Montparnasse leads one to discover things about personages that one might not have guessed: Henri Langlois, who ran the Cinémathèque Française, has a gravestone filled with publicity photos from scores of movies; Jean-Paul Sartre and Simone de Beauvoir share a headstone with only their names and dates; actress Jean Seberg has a simple flat stone grave with a tiny headstone with her name in a badly written script, as though done by an amateur. Many small stones had been left on the sarcophagus lid.

It is no surprise that composer Camille Saint-Saens has a sepulcher with sculpture carved inside. He was an honored figure from the French intelligentsia, but all his pomp doesn’t say as much as the one grave, with no name, and the only thing written on it, in elegant letters is “Le paradis c’est Paris.”

Paradise, but also the underworld.