One of the seven natural wonders of the world, Vietnamese caves

Han Son Cave in Vietnam has not been on the tourist map for a long time. A local farmer discovered it in 1991, but it was not explored and mapped until 2009. Son Son Cave is by far the largest cave on Earth. Its total volume is a staggering 1.35 billion cubic feet. It features an underground jungle with rivers flowing through it. The largest caves are so spacious that clouds often form inside the caves, creating independent underground weather systems.

Han Son Cave is nestled beneath dazzling greenery in Phong Nha-Ke Bang National Park in Quang Binh Province. It’s not easy to get to, but the reward after a few hours of hiking through dense jungle is worth it. Visitors are greeted by a series of caves, each more stunning than the last. The great powers named the first cave "The Great Wall of Vietnam" because the limestone walls towered 300 feet higher. The next room is called "Itami Garden" and features a number of fascinating cave formations, such as towering stalagmites and breathtaking flowstones that look like frozen waterfalls.

Han Son Cave has only been open to tourists since 2013, and there is an annual quota for tourists. For understandable security reasons, the Vietnamese government prohibits personal and informal travel to Son Son Cave. To get there, you need to join a tour group or form your own. Either way, you must book your tour with a licensed Vietnamese company qualified to take visitors into the caves. You are required to use safety equipment such as a hard hat, headlamp and seat belt. Don’t expect it to be a day trip – the tour takes five days. If this sounds too long, it's not. By the end of the tour, you'll be wishing you had more time to explore the underground world you just discovered.