Eisriesenwelt

The world's largest ice cave, high on an Austrian mountainside, where temperatures usually stay below freezing even in summer. Nothing else on this list looks like it, and it rewards the effort. That effort is real. Getting there means a walk of about 20 minutes, a short cable car over the steepest climb, then another 20 minutes on foot to the entrance. The guided tour itself is only a kilometer long, but it takes about an hour and a quarter and involves about 1,400 steps. Wear sturdy shoes and proper winter layers, allow about three hours for the visit, and plan around the season: it opens daily from early May to November 1 and is closed the rest of the year.