Most visitors to Paris have no idea that beneath the City of Light is a dark labyrinth of branching tunnels and abandoned quarries. Paris sits atop massive limestone and gypsum formations that have been quarried for more than a thousand years.
When the mining started, the quarries were outside of town, but over the centuries the city spread and so did the quarries. Eventually Paris ended up with a 1,900-acre underground maze that starts about 15 feet below the streets and ends 120 feet underground. Parisians call the multi-level maze gruyère (Swiss cheese), and that's exactly what a cross-section of the ground beneath their feet looks like.