Eisriesenwelt, an expansive natural cavern outside of Salzburg, Austria, is much more than a standard cave destination. Known as the world's largest ice cave, Eisriesenwelt features amazing rock and ice formations. The subterranean landscapes include stunning frozen waterfalls, semi-translucent pillars of ice. The cave — at least the small portion that tour groups are allowed to enter — has a whimsical feel that makes enduring the below-freezing temps (even during the summertime) worthwhile. Tour season is from May through late October. The ice formations make this arguably the most visually stunning tourist cave in the world. Eisriesenwelt is certainly the best natural underground attraction on our list.