Hang En Cave in Vietnam is the world's third largest cave. At more than 430 feet high and 495 feet wide, it has its own climate, river and jungle.

Explorers can pitch tents on the beach of the crystal-blue river and spend the night, as the trek isn't an easy one. The cave is located six miles into the jungle, and there is no road to get there. Hiking is the only option, and hikers must be willing to wade through rivers ripe with exotic creatures, , Daily Mail reported.