Last Saturday night Stuart and I ventured deep under the Parisian streets to wander through the dark and dank labyrinth that makes up the Paris Catacombs.
Many years ago - 53 million years to be exact - Paris and the surrounding area are a vast swampy plain. This lead to the formation of deep layers of sedimentation rock - rock that was quarried first as open-pits and then as underground tunnels.
Fast forward to the late 18th century and the now heavily built-up Paris had a warren of tunnels and quarries underneath it. Too many to count. There were so many tunnels in fact that there were frequent sink-hole collapses. All further extraction of rock was prohibited for safety reasons.
Coincidentally the Parisian cemeteries were over-flowing. Indeed the Saints-Innocents cemetery had to be shut down it was so full.
The solution? In 1786, benediction and consecration of the first quarry, Tombe-Issoire, lead to it becoming a municipal ossuary. So the Paris Catacombs were born.
For the next 25 years bones were transferred from parochial cemeteries all over Paris under cover of darkness to the new catacombs.
Initially the millions of exhumed skulls and bones were simply thrown into the subterranean quarries, and then later stacked into neat piles and rows by the quarry workers. Gruesome gallery after gruesome gallery soon became a tourist attraction, and now upwards of 550,000 visitors come each year.
It's a fascinating place - if a little macabre.