Photo Credit: Flickr: Javier via wowtravel.com

Our planet isn’t just spectacular above ground. If you haven’t taken the time to discover what lies beneath it, the world’s most magnificent caves offer stunning formations and rivers that make an ideal spot to cool off in on your next vacation. Pictured above, the marble caves of Puerto Rio Tranquilo in Brazil were formed by waves washing against the calcium carbonate walls of the Patagonian Andes for over 6000 years. Reflecting an electric sky blue from the waters of Lake Azure, visitors can embark on a 30-minute guided tour of these breathtaking caves carved into solid marble along the Chile and Argentina border.

Reed Flute Cave, Guilin, China

Photo Credit: sciuniverse.com

Named for its verdant reeds which actually can be made into flutes, this amazing cave was carved from underground karst limestone mountains over millions of years. It’s been among Guilin’s tourist attractions for more than 1200 years.

Ice Cave near Mutnovsky Volcano, Russia

Photo Credit: Florian Wizorek

Photo Credit: Denis Budko

This half-mile long cave was formed by a stream that flows through glacial fields at the bottom of the volcano. This area that’s known for significant precipitation is often covered in 10 feet of snow and ice which blankets mountain streams like this, creating vast, other-worldly-like caves.

Cave of Crystals, Mexico

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The Giant Crystal Cave is connected to the Naica Mine in Chihuahua, Mexico, nearly 1,000 feet into the earth. Its main chamber contains the largest selenite crystals ever found, some of them reaching 36 feet in length, over 13 feet in diameter and weight about 55 tons. The 500,000 year old crystals were formed from underground magma below the cave which maintained the water at a stable temperature of 122 degrees Fahrenheit, saturating it with minerals, including gypsum. This process allowed the selenite crystals to grow to the incredible sizes there today.

Son Doong Cave, Vietnam

Photo Credit: John Spies/Barcroft Media

Son Doong Cave, which means “mountain river cave,” is the largest cave in the world. Located near the Laos-Vietnam border, it was discovered by a local man in 1991 and was formed from Carboniferous/Permian limestone.

Kverkfjöll Glacier Cave, Iceland

Photo Credit: Christian Klepp

This magnificent ice cave in Iceland was formed by the heat of underground hot springs as they flowed toward the sea.