Why limit your adventures to the ground above? Spelunking lets you see the other side of the world, quite literally. Head deep underground to see spectacular stalagmites, cathedral-sized caverns, and eerie glowing creatures. The subterranean world is daunting, and at times spooky, so spelunking is not for the faint of heart. Cavers don safety gear and harnesses and descend into the ground before traversing the network of tunnels, caverns, and caves. Test your metal and get a fresh perspective on the planet.

01Thrihnukagigur Volcanic Cave, Iceland

Thrihnukagigur Volcanic Chamber Anne_Batzelona / Getty Images

Thrihnukagigur Volcanic Cave is a vast, vibrant, and otherworldly-looking magma chamber. Head into the depths of a dormant volcano to marvel at the brightly colored and strangely shaped rock formations. Take a journey in a lift basket down 198 meters underground and watch as the rocks change and see the reds, violets, blues, yellows, and oranges that come from the heat and pressure caused by the eruption of the volcano. Thrihnukagigur last erupted four thousand years ago, so when the volcano went dormant, the magma drained away instead of sealing the chamber.