'Strip the City': Underground danger could bring Paris to collapse

With 300 stations and over 130 miles of track, Paris boasts the second-busiest metro network in all of Europe ... but it could be in grave danger.

Almost half of the city's 10 million daily commuters will use the crowded metro system. One of the stations at risk is the busiest train station in Europe: Gare du Nord (aka Paris Nord) with 28 main line platforms above ground and 4 metro lines underneath. 282 million commuters travel through Gare du Nord every year, and chances are, they don't know about the dangers beneath their feet.

Below Paris are the city's network of ancient quarries, some of which have been filled in for the safety of the 770,000 people that use Gare du Nord every day (though many have not been). In Paris' 13th district, the crumbling quarries put the metro lines in imminent danger.

Geophysicist John Armitage joins engineers from the Inspection Générale des Carrières to examine an abandoned quarry tunnel. "This doesn't look very safe," Armitage notes. The team is looking for cracks, which are a sign that the tunnel could be giving way. They find an "enormous" 2-meter long crack, for which stabilizing walls have been put in to safeguard the cracked rock from completely crumbling.

Space is tight underneath Paris. Metro tunnels run as close as 3 feet below the pavement and 14 feet above the quarries. Paris depends on the stability of the underground world. The city's iconic buildings above ground and the labyrinth of metro lines below are at risk of collapsing into the quarries. The smallest hairline crack in the quarry roof could trigger an avalanche of rock destroying everything in its path and bringing Paris to a standstill.