The tunnels zigzagging through limestone were created to provide building materials for Paris. Then they became the damp home to vast subterranean mushroom farms. Now, deep beneath the city's largest park, the quarry is usually off-limits, deemed too dangerous for regular visitors.

Unescorted spelunkers face a 60-euro ($65) fine, but on Wednesday the 20-meter (66-foot) spiral staircase leading down into the earth from the Bois de Vincennes park was briefly opened to journalists. Temperatures are a constant 13 degrees Celsius (55 Fahrenheit) and humidity hovers around a swampy 90 percent.

Florence Cavailee, who is something of a spokeswoman for the city's underground heritage, says unlike other quarries that have been renovated or opened to tourists, the tunnels underneath Vincennes still look essentially as they did in the 19th century.