This piece on the BBC website seems to hold a number of possibilities for gaming. The Andalusian port city of Cádiz has around sixty kilometres of centuries-old tunnels beneath it. They snake around under the city, connecting numerous parts, and seem just perfect for a scenario setting. Some parts of the tunnels date back millennia, pre-dating the Roman city, and were used by the Phoenicians who founded the city. The Romans added to the network but most date back to the sixteenth and seventeenth centuries, when they were part of the city's defensive works and were also used by smugglers, Jews and others fleeing the Inquisition, and by Rosicrucians.
The network is being explored, slowly, by the Association of Investigators of the Cadiz Underground. What might they unearth?