Inside The World's Biggest Cave System

1. A torch-wielding man casts a tiny shadow as he stands in the vastness of the world's biggest cave, the Hang Son Doong cave in Vietnam.

2. At more than 650 feet high, almost 200 feet wide and 3.1 miles long, the Hang Son Doong cave in Vietnam is so big it has its own river, jungle and climate.

3. Australian photographer John Spies spent a week photographing the natural wonder and the handful of tourists who pay up to 3300 each to marvel at its beauty.

4. "With ceilings towering over 200 meters high in places, the cave is a humbling and belittling experience," said John, who has lived in Thailand since 1977 and runs the Cave Lodge guesthouse.

5. British cavers first entered Hang Son Doong in 2009 but it has only been open to tourists since late last year.

6. The entrance to the cave is quite small, and mist from the cave, caused by the cooler air inside meeting the hot air outside, rises into the surrounding forest. Visitors must descend 260 feet down a steep wall with the use of harnesses and ropes.

7. A river flows through the Hang Son Doong cave.

8. Photographer John Spies: "It is amazing to be 3-4km inside the cave and have daylight illuminate the cave formations. The dimensions of the cave are incredible - and to camp for five nights in the biggest cave in the world is not something most of get to do in our lifetime."

9. The cave tours are run by Ho Khanh, a local farmer who officially discovered the cave entrance in 1991.

10. The cave is five times the size of Malaysia's Deer Cave, which previously held the title of the world's biggest cave.

11. It was formed between 2 and 5 million years ago by river water eroding away limestone underneath the mountain above.

12. The huge temperature difference creates moving clouds of mist, especially near the two karst windows, which gives the cave its magically surreal atmosphere.

13. But this is no easy trip and visitors have to be physically fit and able to cope with the demanding conditions inside.

14. Photographer John Spies: "There are some sections you could safely jog through. However, there are some more challenging sections where you cross massive boulder-piles and pass under house-sized chunks of limestone.

15. John Spies: I have spent many nights camped deep inside dark caves and it makes you feels very vulnerable, being so far from light. With our cave camps set on sandy beaches near the karst windows, it is easy to forget you are many kilometres underground."

16. Cavers gather around their site for a rest.

17. A caver at the entrance of the Hang Son Doong cave in Quang Binh Province, Vietnam.

18. Cavers near one of the entrances of the Hang Son Doong cave.

19. A water body inside the Hang Son Doong cave.

20. A caver stands outside one of the entrance of the Hang Son Doong cave in Quang Binh Province, Vietnam.

21. Taking a rest inside the huge cave.

22. A caver stands near the entrance of the Hang Son Doong cave in Quang Binh Province, Vietnam.

23. A caver sits inside the Hang Son Doong cave in Quang Binh Province, Vietnam.