(CNN) — Thanks to discoveries made during a recent expedition in Vietnam , it appears the world's largest cave, Son Doong, is even bigger than previously thought.

Last month, a trio of British divers -- the same divers who aided in the rescue of the trapped soccer team in Thailand in 2018 -- ventured to Phong Nha-Ke Bang National Park, in the jungle-filled Quang Binh Province of central Vietnam, to explore the cave's waterways.

During the groundbreaking dive, they discovered an underwater tunnel that connects Son Doong (meaning "Mountain River Cave") with another enormous cave called Hang Thung.

Son Doong currently measures a total of 38.5 million cubic meters (about 1.35 billion cubic feet). When it's officially connected with Thung Cave, it will add an additional 1.6 million cubic meters in volume.

"It would be like someone found a lump on top of Mount Everest, making it another 1,000 meters higher," says Howard Limbert, technical advisor of the Quang Binh-based Oxalis adventure tour company and one of the cave experts who helped organize the dive.

He tells CNN Travel , "Any cave in the world will be able to fit comfortably inside Song Dong when it's connected -- it's just outrageous in size."

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Diving in Son Doong

British divers Jason Mallinson, Rick Stanton and Chris Jewell -- who helped rescue the trapped soccer team in Thailand in 2018 -- recently dived in Vietnam's Son Doon Cave. courtesy Oxalis Adventure

Oxalis, which is the only company licensed to bring travelers into Son Doong Cave, invited the British divers -- Jason Mallinson, Rick Stanton and Chris Jewell -- to visit the cave following the Thai rescue expedition.

"The divers did an amazing job rescuing the children in Thailand. We invited them on a trip to Son Doong to thank them for their great effort," says Limbert.

"They wanted to do something interesting during the trip, so we came up with this idea of diving Son Doong, which had never been done before."

Limbert says that the Oxalis team already knew that the water from Son Doong joined Thung Cave, through dye-testing, but no human has ever gone into these subterranean rivers.

During the mission, divers were able to reach a depth of about 78 meters while diving on air (oxygen and nitrogen) before turning around.

"When the divers reached 78 meters, they plumbed the depth below using a line and a lead weight. Hence, they believe the offshoot reaches a depth of 120 meters and continues for about 1 kilometer," explains Limbert.

The team wasn't expecting the tunnels to be so deep, because the other caves in the area are quite shallow.

"Now that we know how deep it is, we'll bring the special gases [oxygen-helium mixture] with us next time to enable long, deep dives," adds Limbert.

The divers plan to return in April again next year. This is the best time of year to dive, because water levels are relatively low, and visibility is better than usual -- though still only about one to two meters.

"I think it's incredible that something as important as the world's largest cave is still being explored and better understood," says Limbert.

"No one had ever set foot inside Son Doong until 2009 ... and this latest discovery shows there are still an awful lot of things to uncover on this planet. It's really exciting."

Uncovering the cave

l e v a r t

One of the world's most precious natural wonders, Son Doong was inadvertently discovered by Vietnam resident Ho Khanh in 1990.

"While hunting in the jungle, Khanh came across the opening. He felt a blast of wind and heard the rush of a river inside... But after he left, he couldn't find it again, because it's surrounded by foliage," says Limbert, who was part of the British Cave Research Association (BCRA) team that first set foot inside Son Doong.

"Khanh spent many, many years trying to rediscover the mouth of this cave and, finally, in 2009, he led us there. We realized right away that it was major."

After the BCRA explored and measured the cave, they proclaimed it the largest in the world in 2010 -- so big that you could fit a New York City block with 40-story skyscrapers inside it, according to Oxalis's estimations.

Within the immense limestone cavern, there are various microclimates and diverse scenery including two jungles, made possible by dolines -- openings created by collapsed ceilings -- where sunlight streams in.

"So far we've only explored about 30% of Phong Nha-Ke Bang National Park, so there's a lot more to be discovered," says Limbert.

See it for yourself

Hang Son Doong in Vietnam is the largest known cave in the world. Photographer Ryan Deboodt took a drone inside to capture this spectacular footage.

Opened to tourists in 2013, the three-million-year-old river cave is easily one of the most beautiful places on Earth.

"People love it because they've never done anything like it before," says Limbert. "Obviously, you have amazing stalactites and the world's largest stalagmites (up to 80 meters high). They are so spectacular."

To explore, travelers must book a guided excursion through Oxalis.

From river crossings to jungle treks and a harnessed ascent up the 80-meter-high "Great Wall of Vietnam" to nightly campsites, the four-day experience traverses 25 kilometers (about 15.5 miles) of jungle trails and nine kilometers inside the caves.

"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."

It's certainly physically challenging, but Limbert says the trek is accessible to anyone who is reasonably fit and doesn't mind getting muddy. And you will get muddy.

Into the wild

During the mission, divers were able to reach a depth of about 78 meters while diving on air (oxygen and nitrogen). courtesy Oxalis Adventure

The first 1.5 days are spent trekking through Hang En Cave -- the third-largest cave in the world -- to reach ultra-remote Son Doong. The next few days are spent exploring the otherworldly landscapes of this remarkable cave, sleeping at a different campsite each night.

Among the many highlights of the trip, Limbert says there's one experience in particular that everyone goes "wild about."

"There's a passage that has beautiful, 400-million-year-old fossils. We take travelers to see these fossils and, once you're down there in the dark, there's a place where you can go swimming," says Limbert.

"Everyone loves the experience of swimming in the dark in the cave -- it's a really unusual experience."

Available from January to August, the US$2,990 tours are limited to just 10 people at a time, with a maximum of 1,000 travelers each year. Accompanying each group is a 27-person strong support team, including porters, cooks, safety assistance, BCRA-trained guides and caving experts.