THE world's largest cave transports you back to a time long before humans ever existed with its own underground weather system, mysterious river and rain forest.
Hang Son Doong in Vietnam is considered to be the biggest cave by volume in the world which no human stepped inside of up until 14 years ago.
The cave is located in Phong Nha Ke-Bang National park which has cave systems stretching over 120 miles, and many of them remain unexplored.
Hang Son Doong translates to "Mountain River Cave" from Vietnamese.
It is 655 feet high, 490 feet wide and has a length of at least four miles.
To put it into perspective, the largest passage of this cave could have a Boeing 747 fly through it.
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And the underground chamber is so vast, it could fit several forty-storey skyscrapers inside.
But what makes this cave special isn't the size, but the unique hidden underground world inside the cave itself.
Hang Son Doong has many complex and giant stalactites, rain forests growing inside, its own weather and unknown animal species.
And the underground river which remains a mystery as no explorer has reached the end of it yet.
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This river has formed the cave's distinctive environment and ecosystem.
Son Doong is its own planet that houses seven new species of fish, spider, scorpion, shrimp, wood lice and plankton.
These unique animals are completely white and without eyes as a evolutionary consequence of living in complete darkness.
Within the depths of the cave are huge stalagmites, waterfalls, intricate passages, and tall jungle trees.
The stalagmite named Hope and Vision could well be the world's biggest of its kind.
At the whopping height of 262 feet, it is a lot taller than the current title holder -a 229 feet stalagmite in Zhi Jin Cave, China.
Although Son Doong is pitch dark in most places, the collapsed ceiling of the cave created a big opening to the outside which invites plenty of light in.
The cave passages are illuminated by two sinkholes that can flood them with beams of spectacular sunlight.
But the visibility inside remains restricted to one mile due to incredible clouds created by the cave's own weather system.
Visitors describe Son Doong as otherworldly, reminding them of Pandora planet from James Cameron's movie Avatar.
Howard Limbert, leader of the caving team that mapped Son Doong, described the cave, saying: "There is nowhere like this place anywhere in the world.
"It is not just the size – though it does matter (as they say) – but the variety of unusual and amazing locations within the cave, such as swimming pools in the dark and 400-million-year-old fossils."
Next to the underground river, there is a passage imprinted with corals which have been fossilised hundreds of million years ago.
There is also an underground lake with 20 degrees Celsius temperatures that visitor swim in.
During the rainy season, another lake appears in the cave filled with beautiful jade-green water.
Deep inside the cave, there is a primeval forest with trees growing over 90 feet tall.
Son Doong is estimated to be between two and five million years old, but no human stepped inside the cave up until 2009.
The cave was originally discovered by a local farmer Ho Khann in 1991.
On his search for agarwood-an expensive plant- in the woods, Khann encountered a thunderstorm and had to seek shelter.
That's how he found a small cave entrance below a cliff, which had mist blowing out of it and the sounds of the river roaring inside.
But Khann didn't go inside and the cave would remain undiscovered for the next 18 years.
It wasn't until the members of the British Vietnam Cave Expedition Team inquired about the cave from the farmer that it was explored for the first time.
On April 7 2009, Howard Limbert and his team, together with the local farmer Ho Khann, led an expedition into the cave.
"We were the first people to enter the cave," Limbert told Lonely Planet.
"There is no evidence that anyone lived in Son Doong in the past.
"Usually it’s easy to see this in a cave. Also, all the entrances and exits involve near-vertical descents of at least 85m using technical equipment."
But the access to the cave remains exclusive to a small number of people.
Only 1,000 visitors are allowed to enter each year between the end of January and August.
And the privilege of witnessing the hidden beauty of Son Doong can cost £2,380 for a four-day-tour.
In 2013, the Guinness Book of World Records named Hang Son Doong as the largest cave in the world.
With a volume of 38.5 million cubic metres, it is five times bigger than the Deer Cave in Malaysia, which was previously deemed the largest natural cave.
And as Son Doong isn't fully explored, there is a possibility that it may get bigger still.
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