World's Coolest Underground Wonders

Planet Earth is a mysterious place that never ceases to amaze. You won't have to look far to find some natural peculiarity for which science sometimes offers a theory. However, many locations are still a complete enigma.

You can fly around the world and dive deep under the sea – and you will see some absolutely astonishing wonders – but there are cities hidden right beneath your feet that will amaze you even more.[slideshow:101518]

Some are basically complex tunnels or cave systems which people have transformed over the years into livable dwellings. There are many cave hotels you can stay in all over the world as well.

The world map can be deceiving because it creates an illusion that there is nothing left on Earth to discover. But there are places that have been seen by few people. One example is a sweltering cavern in Mexico filled with crystals as tall as apartment buildings and an underground lake that is somehow so clear that its water make you dizzy.

Some of the cool underground wonders on this list are entirely a creation of Mother Nature, while others have been formed thanks to man's ambitions and capabilities.

Click here for 16 of the world's coolest underground wonders

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