"Empire of the dead": Paris's catacombs hold the remains of some six million people.

The air was cool and exuded a musty, almost earthy odor. The passageway with squat ceilings no more than six feet high was lit by small dim lights, each bright enough to see only a few feet ahead. Small rivulets of water seeped from the walls to make the uneven stone floor slippery. Along both sides were thousands of skulls and bones, stacked in dense chest-high rows.

Each passage led to rooms with walls lined with stacks of human femurs and skulls, arranged with workmanlike precision. The only greeting along the bone-filled tunnels came from the gaping grins of the long-ago dead.

It is said the bones of six million souls reside here. These are the catacombs that lie under the bustling streets of Paris, France.

Continuously inhabited for over 2,500 years, Paris is the definition of an old city. For hundreds of years, the dead were buried in graveyards sprinkled on its outskirts. But as the city expanded, it grew around and engulfed these dozens of cemeteries. By the Middle Ages, Paris was a very dirty place where open-air markets coexisted alongside cemeteries containing the shallow pits of mass graves.

In the late 18th century, to clean up the city and save it from itself, it was decided to disinter bodies from many of the cemeteries and respectfully transfer them to the catacombs. There, the bones and skulls were neatly stacked and marked with placards of the cemeteries from which they came. The newly vacated space was then used for housing the ever-expanding population of the city. The transfer of the residents of about three dozen cemeteries, some 1,000 years old, continued intermittently for the next 100 years.

The catacombs are remnants of limestone mines first quarried by the Romans to construct roads, bridges, monuments and buildings. Over the next millennium, the Roman open pit mines developed into a series of underground mines, but were abandoned by the 13th century. The labyrinth of limestone tunnels was a perfect place to move the remains of so many millions of people.

Descending 75 feet from street level down a cramped spiral staircase, the walking tour continues under a stone arch chiseled, “ARRÈTE! C’EST ICI L’EMPIRE DE LA MORT.” (“STOP! HERE IS THE EMPIRE OF THE DEAD.”)

It only gets better from there. The tour of the Paris catacombs is a fascinating part of French history – but it’s not for the squeamish.

The entrance to the catacombs is in southern Paris on 1, Avenue du Colonel-Henri-Roi-Tanguy. The metro is right across the street. For additional information, see the Catacombs website.