In the late 18th century, the former Tombe-Issoire quarry galleries under the plain of Montrouge were converted into an underground cemetery. The city was then facing real health problems linked to its growing population and the inadequate old Roman cemeteries. That situation inspired the idea for the biggest ossuary in the world, the Paris Catacombs.
With its average temperature of 14°C, both coolness and spine tingles are guaranteed. Walk down the 131 stairs to the heart of the catacombs, 20 metres below the surface. The circuit is 1.7 kilometres long and recounts the history of subterranean Paris. The official tour covers but a small fraction of the labyrinthine structure, with its 300+ kilometres of galleries under the capital city, allowing it to retain a great sense of mystery.
Because close to 500,000 visitors come each year, I have a tip for you: buy a quick-access ticket to avoid waiting in line under the hot, hot sun!