Fewer people have seen inside this incredible place than have reached the summit of Mt Everest. And the wait to visit just got longer.
This is the largest cave in the world, and as a few lucky tourists have discovered, it is spectacular.
Situated in the Phong Nha-Kẻ Bàng National Park, in the central Vietnamese province of Quảng Bình, the Son Doong cave is about nine kilometres long and 200 metres high - big enough for an entire New York City block, complete with 40-storey skyscrapers, to fit inside.
Inside the cave are incredible, smaller cave systems, as well as a river and a jungle. The cave and its surroundings are also widely noted for having a Lord of the Rings-esque quality about them, with otherworldly rock formations, rich green moss and eerie displays of natural light.
The discovery of the Son Doong came to the attention of the world just seven years ago, and since then plenty of tourists have signed up for a trek throughout it.
But it's become so popular that if you were planning on seeing the incredible attraction in the next couple of years, you've already missed out.
According to Lonely Planet, Oxalis Adventure Tours - the only tour operator that takes travellers on treks inside the cave - put tickets for 2017 tours on sale last week and they sold out in just 20 hours.
Now eager tourists will have to wait until after next year for the chance to see inside Son Doong.
The incredible cave was discovered in 1991 by local man named Ho Khanh, but because of the steep descent at the entrance to the cave - as well as the threatening rumble of the river inside - local people avoided going inside.
It wasn't until scientists from the British Cave Research Association ventured into the cave in 2009 that its existence was revealed to the world.
Since it opened for tours in 2013, a limited number of tourists have been able to explore the cave on treks organised by Oxalis Adventure Tours.
Only 500 to 600 people are allowed inside during the January to August season, and the limited visits is because of safety and conservation concerns, Vietnam's Thanh Nien News reported.
According to Oxalis, fewer people have seen the inside of the cave than have reached the summit of Mt Everest.
Tickets are sold to the first applicants who meet strict health requirements, and anyone who can't make it to the entrance of the cave on their own are turned away. It is rated as the most difficult of all Oxalis' adventure and expedition tours in Vietnam.
Five-day, four-night trips include a total trek of 50 kilometres, and cost about $4138, or 67 million dong, per person.