This is the official entrance to the 3,000km (1,864-mile) tunnel network that runs under much of the city. With public burial pits overflowing in the era of the Revolutionary Terror, the bones of six million people were transferred to the catacombes.
The bones of Marat, Robespierre and their cronies are packed in with wall upon wall of their fellow citizens. A damp, cramped tunnel takes you through a series of galleries before you reach the ossuary, the entrance to which is announced by a sign engraved in the stone: 'Stop! This is the empire of death.'
The tour lasts approximately 45 minutes and the temperature in the tunnels is 14°C.