The Paris Catacombs has a very discreet entrance. It looks like a small garden shed attached to one of the city’s municipal buildings. I would have walked right past it were it not for the hundreds of people waiting patiently in line to enter. Even on a cold wet day the line stretched back almost 100 meters and I was glad to see our guide Karlie standing a the head of the line.

“Entrance is limited to just 200 people at a time,” Karlie explained. “So once the Catacombs are full you have to wait for someone to come out”. Luckily this tour had exclusive skip the line access which meant we left the damp weather behind as soon as the group was ready.

Passing the cramped ticket booth we descended a narrow spiral staircase – an exhausting 130 steps – to find ourselves in a small room almost 20 meters below where we’d just been standing. “We’ve just entered the old quarries of Paris,” Karlie began, explaining the history behind an area of hidden Paris that even most Parisians know little about.

After filling us in on what exists underneath the city Karlie introduced us to the Catacombs themselves. Overflowing graves, poor burials and centuries of poor health, disease and war led to what sound like unbearable living conditions in the city. The words “stench”, “partially decayed” and “unimaginable” were used on several occasions to describe Paris in the 1700s. Although on paper it doesn’t sound like the most family-friendly tour, the younger members of the tour were hanging on Karlie’s every word as she promised more gory details once we got inside.

After a 500m walk through the subterranean maze we reached the Catacombs where Karlie pointed out some fascinating but easy-to-miss features. One of our best finds was a series of carvings of Fort Mahon, made by a former worker who carved a map of the fort straight into the rock from memory.

As we rounded a corner, the atmosphere changed dramatically – dim, stark walls giving way to a passageway decorated with black and white symbols. People crowded around to take photos of a plaque above the door that read "Arrette! C’est ici l’empire de la mort." Stop! Here is the empire of the dead.

Lit only by the occasional light, the Catacombs were fairly dark. After a few tense minutes of groping through the darkness, my eyes adjusted and my gaze settled on the morbid scene. Stacked on either side, stretching for hundreds of meters were walls lined with bones. Leg bones were arranged in a careful fashion with skulls placed at intervals, either in a repeating pattern or with “artistic tendency” as Karlie put it – a heart made of a dozen skulls, a real-life skull and cross bones, arches, circles and triangles all outlined by the remains of dead Parisians.

To some, this scene symbolises a measure of respect that the dead were denied in their previous burials and to others it is the natural result of man’s creativity – macabre as it is. One thing is certain though – the creations are as fascinating as they are disturbing.

“No one believes me until they see this,” said Karlie. “But there are between 5 million and 6 million people down here”. Yet standing in the Catacombs it wasn’t hard to believe. In some places bones were stacked to the ceiling – over two meters high – and in spots they reached several metres in depth too. And the scene continued for over a mile, every twist and turn of one passageway leading to another, each one a line of neatly stacked remains disappearing into the darkness beyond.

As we traced our route Karlie filled in the blanks of our imagination with tales of souls buried in the Catacombs – lives ended by violent revolutions and razor-sharp guillotines. We walk for an hour, listening to tales and plaque translations. Captivated, the time flew and we were startled when suddenly we found ourselves once again blinking in the sunlight, Karlie’s parting words ringing in our ears.

“As you walk around Paris think about what lies beneath you,” she urged. “I’m sure you’ll never see the city in the same way again".

For more information on our Skip the Line: Catacombs tour click here.