World's Coolest Underground Wonders

Planet Earth is a mysterious place that never ceases to amaze. 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. 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.

Inside a magnificent cave is the famous geological formation named Enchanted Well – or Poço Encatado. The sunlight is reflected by minerals in the water, especially magnesium, and creates a unique spectacle. The best time to go is between April and September if you want to see the alignment phenomenon – the rays hit the deep ground of the blue well inside the cave.

The surreal qualities of this cave have a lot to do with the artificial lighting, but the natural limestone is over 180 million years old and is filled with many strange and surreal shapes. The colors simply add to the fun. The cave got its name from the type of reed growing outside.

Chu Chi Tunnels are part of a massive war museum that shows the underground life of Vietnamese soldiers back in 1948, according to Vietnam Guide. The historical site comprises more than 75 miles of underground tunnels with several trapdoors, living areas, kitchens, storage facilities, armory, hospitals, and command centers. The tunnels were also used and expanded during the war with the U.S. by adding air filtration systems.

The cave is almost 1,000 feet below the earth's surface. Discovered in 2000, much of the cave still hasn't been explored due to the extreme conditions. With temperatures that can reach 136 degrees and 90 to 99 percent humidity, people can only endure 10 minutes in the caves at a time without protection. The Cave of the Crystals is home to some of the largest natural crystals ever found.

This can be your personal underground suite – for $850 per night for two people. This is how much the experience in the largest, deepest, darkest, oldest, quietest motel room in the world costs. The caverns and walls are over 65 million years old. The room is 220 feet below the surface (22 stories underground) and it's accessed via an elevator. It is also 200 feet wide and 400 feet long, with a 70 foot ceiling.

This tourist attraction is a real museum of salt mining in Transylvania. The absolute age attributed to the salt deposits is 13.6-13.4 million years. The subsequent tectonic evolution of the basin determined the formation of long folds. Some of the salt is concentrated in diamond-shaped seeds. The salt mine includes an amphitheater, children's play area, table tennis accessories, a sports ground, a place for mini golf, and a track for bowling.

This extraordinary system of limestone caves includes collapsed dolines, some four miles of underground passages with a total depth of more than 660 feet, many waterfalls, and one of the largest known underground chambers, according to UNESCO. The site, located in the Kras region, is one of the most famous in the world for the study of karstic phenomena.

The highlight of this 5-mile long subterranean river system, the longest underground river in the world, is that it flows directly into the sea, with its brackish lower half subjected to tidal influence, distinguishing it as a significant natural global phenomenon, according to UNESCO. The river's cave presents extraordinary rock formations. You can see hundreds of bats, huge monitor lizards, sea snakes, and other wildlife.

Qin Shihuang (259-210 B.C.) wanted to conquer death. To achieve immortality, he had a giant tomb—a vast underground city guarded by a life-size terracotta army including warriors, infantrymen, horses, chariots – built, according to Khan Academy. An extensive fortress was uncovered, which has gardens and stables, bronze ritual vessels, jade jewelry, and many gold and silver ornaments.

A massive rock rises at an elevation of 1,118 feet. It is home to the world's most acclaimed cave complex of magnificent Buddha images and rock paintings constructed from around 2nd century B.C. up to the 18th century A.D. This cave monastery, with its five sanctuaries and 157 statues, is the largest, best-preserved cave-temple complex in Sri Lanka.

Few natural marvels in the world can compare to an awe-inspiring tour through the glow worm caves in Waitomo. It is difficult to capture the magnificent beauty that occurs in these caves. The glowworms in this particular cave are a species around the same size of a mosquito found exclusively in New Zealand. As you paddle through the caves, the glowworms light the ceiling above you, twinkling like the night sky.

The underground city is one of five inter-connected complexes beneath the surface with a total estimated capacity of 100,000 people. Derinkuyu is the deepest in the historical Cappadocia region – about 280 feet. There are about 600 outside doors to the city, hidden in the courtyards of surface dwellings, according to Turkish Heritage Travel.

The City of Lights sits above over 200 miles of tunnels. Some are even lined to the ceiling with skulls and bones. The city was way too overcrowded by the 17th century and the solution to overflowing cemeteries was to bury the corpses in the tunnels that have existed since the 13th century, according to Smithsonian Magazine. About 7 million bodies were moved into the catacombs. The oldest date back to 1,200 years ago.

The mining town is one of the most unique places in all of Australia. It's often referred to as the country's underground town because 80 percent of the people live as a subterranean community, largely to stay out of the summer's 120-degree temperatures. The town is one of the hottest places on Earth.

Istanbul is hiding hundreds of ancient cisterns underneath its streets and houses. Of the two that are open to the public, the Basilica Cistern (Yerebatan Sarnıcı) is the largest and the city's most unusual tourist attraction. You can take a stroll in the forest of hundreds of marble columns, according to Istanbul Trails. This enormous underground water container was built in 532 to meet the water needs of the Great Palace.

Imagine going down into a massive magma chamber of a dormant volcano. Iceland is the only place in the world where a tour like this exists, according to Guide to Iceland. This journey toward the center of the planet covers 650 feet and takes about six minutes. Strong heat and pressure from Þríhnúkagígur's most recent eruption, which occurred over 4,000 years ago, gave the rocks their stunning colors and texture.