Grotte de Médous

Grottes de Médous is a limestone cave system near Bagnères-de-Bigorre in the Hautes-Pyrénées, opened to the public in 1951 after cavers broke through a clay blockage in 1945 that had sealed off more than 2 km of passage and revealed an underground river running beneath the mountain. The tour winds through galleries of stalactites, stalagmites, and calcite draperies, including the Jardin des Orchidées with its delicate eccentric formations and the cavernous Salle du Grand Chaos, choked with 45-meter rockfall, before you board a boat to glide across the underground river itself, which dye tests decades ago confirmed is fed by the Adour.

Tours run 45 minutes to an hour, mixing walking on stairs with the boat crossing, so it isn't workable for wheelchair users or anyone with significant mobility limits, and strollers aren't allowed (bring a baby carrier instead). It's a constant 13°C underground regardless of the season outside, so pack a jacket and closed-toe shoes even in summer. Small dogs can join if you're prepared to carry them through the boat section, and flash photography is banned to protect the formations. Outside the peak summer months, departures run on a scheduled basis and need at least two adults to go ahead, so it's worth checking timing before you show up in shoulder season.