Beneath its boundless boulevards, charming buildings and iconic landmarks lies Paris' darkest spectacle – Les Catacombes. A 3,000km (1,864-mile) tunnel network of skulls and bones spans most of the city. In 1785 during the Revolutionary Terror, due to public burial pits overflowing, it was decided that the skeleltons of six million deceased be stored in the catacombes. The 20 metre descent below street-level leads you down to the dark passages packed with these human remains. You walk 2km finally reaching the ossuary, where just in case you needed reminding, a sign screams: 'Stop! This is the empire of death.'