(CNN) — Vietnam already has a worldwide reputation for its food, beaches, ancient history and diverse landscapes. And, over the past decade, it's also emerged as one of the world's leading destinations for caving.
Quang Binh province in central Vietnam is particularly famous, thanks to the UNESCO-protected Phong Nha-Ke Bang National Park -- also known as the Kingdom of Caves. Last year alone, the region welcomed more than 3.9 million visitors, representing an 18.2% uptick compared with the year prior. "Vietnam has some of the best caves in this world. The temperatures are pleasant all year, the jungle scenery is beautiful, there are stunning formations... It's all untouched and pristine -- nothing's been damaged," Howard Limbert, technical advisor at the Quang Binh-based Oxalis adventure tour company, tells CNN Travel.
"My team [the British Cave Research Association] has explored more than 500 caves in Vietnam. But we've still only explored about 30% of this area. There are many, many more caves to be discovered."
For those keen to explore Vietnam's most mesmerizing underground worlds, here are a few worthy adventures to consider:
Son Doong and Hang En
Divers have discovered an underwater tunnel that connects Vietnam's Son Doong -- the world's largest cave -- with another enormous cave.
Said to be the largest cave in the world, Son Doong currently stretches across 38.5 million cubic meters (about 1.35 billion cubic feet) -- so big, it even has its own jungle inside.
Following a recent discovery of an underwater tunnel that connects Son Doong with neighboring Hang Thung cave, it could be an additional 1.6 million cubic meters larger than previously thought.
Oxalis, in partnership with the British Cave Research Association, has exclusive access to the cave, bringing small groups of six to 10 people on four-day treks during dry season from January to August every year.
On the first day, travelers hike through the jungle for about an hour before stopping for lunch in the village of Ban Doong.
After a few more hours of trekking, the group sets up camp inside Hang En Cave (the third largest in Vietnam) where you can swim in underground rivers and refuel with Vietnamese cuisine.
The next day, hikers continue the journey to reach remote Son Doong Cave where they'll don safety harnesses then use ropes to descend down an 80-meter-tall rock wall.
Once inside, travelers spend the next two days exploring the immense cave's ancient fossils, underground jungle, stalagmites and underground rivers.
The journey culminates in an epic scramble up the "Great Wall of Vietnam" -- an 80-meter-tall rock wall that requires a mix of scrambling and climbing of ropes and ladders.
"This is definitely not just a stroll. There are lots of river crossings, superb jungles, mountains and cliffs all around, plus lots of wildlife ... like birds and monkeys that are endemic to this area," says Limbert. "A lot of people find the scenery as spectacular as the cave itself."
Hang Va and Nuoc Nut
Hang Va is known for its rare calcite rock formations, or tower cones.
courtesy Oxalis Adventures/Ryan Deboodt
A two-in-one combo, Oxalis also offers two-day tours of Hang Va and Nuoc Nut caves.
On the first day, you'll take a two-kilometer trek to Nuoc Nut cave, where an incredible underwater river awaits inside.
A 100-meter swim through the river passage connects you to the entrance of Hang Va cave, where travelers camp for the night.
While more accessible than Son Doong, the cave expedition is still an adventure with lots of opportunities to hike through thick jungle foliage, climb over rocks, traverse subterranean rivers and admire surreal stalactites and stalagmites.
Hang Va, in particular, is known for its extraordinary stalagmite field.
"It has more than 100 rare calcite rock formations called tower cones," says Limbert. "They're essentially stalagmites that formed underwater -- and they're incredibly rare!"
Tu Lan Cave System
Hang Tien's enchanting rock formations look like terraced rice paddles.
courtesy Oxalis Adventure/Ryan Deboodt
Comprising more than 10 caves in Quang Binh province, the Tu Lan Cave System has skyrocketed in popularity since starring in 2017 blockbuster "Kong: Skull Island."
"A lot of people come because of the movie, but they find that the scenery is even more beautiful than they imagined," says Limbert.
"All the campsites are near lakes or waterfalls, so it's brilliant for swimming in the warm weather. On our Tu Lan tours, you go through one cave entrance, exit into another part of the jungle then back into another cave."
Oxalis provides one- to four-day tours through the region for flexibility and limits visitors to keep the region pristine.
"In this area, we keep it really controlled and you rarely see other people. You'll have a cave to yourself," says Limbert.
Within Tu Lan, one of the most popular excursions is the two-day tour through Hang Tien (or the Cave of Fairies) -- named after enchanting rock formations, which look like terraced rice paddies, and magical swirling patterns that appear on the limestone walls.
Crossing through two caves (Hang Tien 1 and Hang Tien 2), the tour takes travelers on a jungle trek before dipping into the enormous, dry caves which are a combined 5.5 kilometers long and 100 meters tall at their peak.
"These caves are drier than many of the other caves in the area, so they tend to be popular with travelers who don't want to swim on the tour," says Limbert. "They're just massive -- you will never get claustrophobic in these two caves."
Another beauty within Phong Nha Ke Bang National Park is Paradise Cave (also known as Thien Duong Cave).
A local man stumbled upon the entrance -- just a hole in a rock, hidden behind thick foliage -- in 2005.
Soon after, the British Cave Research Association explored the cave's interiors and remarked on its grandeur and beauty, later opening it up to travelers.
It's now one of the most frequently visited caves in Vietnam -- the otherworldly stalactites, which tumble from the cavern ceiling like waterfalls, and sculptural columns never fail to impress visitors.
Nor does the size: thought to be the longest dry cave in Asia, at more than 31 kilometers long, the limestone cavern is large enough to house a jumbo jet.
Travelers can experience it two ways: a one-kilometer walk via a network of illuminated wooden stairs or an overnight excursion that includes a seven-kilometer hike.
"I always recommend the seven-kilometer trek because you will have much more of an experience, spending at least six hours trekking deep inside the cave itself," Loc Nyugen, founder of private tour operator Xinchao, tells CNN Travel. "This is a really unforgettable experience."
The journey to the hidden Tam Coc, "Three Caves," is as unique an experience as the visit inside the cave.
courtesy Vietnam Tourism Board/Christian Berg
Beyond the popular Quang Binh province is the UNESCO-listed Trang An Scenic Landscape Complex. Located in the northern Ninh Bình Province, it's another place to find beautiful caves.
"What I love about caves in Vietnam is that each one has a different character," says Nguyen.
"The journey is also part of the experience. Sometimes it's more about how you reach the caves that makes it really special -- maybe you walk, hike, swim or boat there. It adds to the experience."
That's particularly true of Tam Coc, meaning "three caves," within Trang An. This trio of caves -- Hang Ca, Hang Hai, and Hang Ba -- can all be accessed on a rowboat tour starting from Van Lam village.
A gentle, three-kilometer cruise down the Ngo Dong River passes by glistening rice paddies and karst formations before finally reaching the caves.
Hang Ca Cave is the largest of the three caves stretching more than 120 meters long, while Hang Ba is the smaller at just 45 meters long.
"Since you just take a boat through the caves, it's very pleasant and easy to reach," says Nguyen.
"The best time to go is March through May, when the farmers will be growing rice in the field -- it's a beautiful sight, just a sea of bright green and yellow."
Halong Bay has its fair share of scenic caves.
courtesy Vietnam Tourism Board/Christian Berg
Halong Bay might be best known for its thousands of limestone islands, rising out of the water. But there are also lots of caves too -- at least 59 listed to date.
The most famous is Sung Sot, also known as Surprise Cave. Located on Bo Hon Island within Halong Bay, the cave was first discovered in 1901 by French explorers.
Sprawling across 10,000 square meters, the cave is wide and spacious with an incredible array of stalactites and stalagmites, many of which take the shape of flora and fauna.
"There's actually a lot of folklore and myths around the shapes and patterns of the rocks, which can be fun for travelers," says Nguyen.
"Most boat tours include Sung Sot on the itinerary, stopping by the entrance and letting you off to explore for a couple of hours."
While much smaller in size than Sung Sot, Luon Cave feels like a gateway to a secret world within Halong Bay.
This arc-shaped grotto, which is just 60 meters long and four meters high, can only be accessed by kayak or small boat.
When the water's high, you have to crouch to get through the low entrance.
Inside, the cave looks eerie and feels cool, with clear blue water below and another narrow exit straight ahead.
On the other side of the cavern, a peaceful lagoon is surrounded by limestone cliffs where tribes of monkeys complete the picture.