A secret laboratory 800m below the Pyrenees

In Canfranc, the town where Nazis once smuggled 86 tons of stolen gold, scientists conduct experiments on dark matter that could transform the field of physics.

Join more than three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "The Essential List". A handpicked selection of stories from BBC Future, Culture, Capital and Travel, delivered to your inbox every Friday.