Twenty-two kilometres northwest of Ax-les-Thermes on the N20 near Ussat-les-Bains, Europe’s largest cave system burrows its way through the soft limestone rock beneath the Pyrenees’ peaks. Guided tours take in more than 200 stalactite-lined tunnels, grottoes and galleries, including a sandy expanse known as the Sahara Desert, and limestone columns variously resembling a mammoth, a wizard and the Virgin Mary.
Standard tours last two hours, but longer 'randonnée spéléologique' expeditions are available for the adventurous (and non-claustrophobic).