In pictures: Inside Hang Son Doong, the world's largest caves in Vietnam

SonDoong Cave

At more than 200m high, 150m wide and 5km long, the Hang Son Doong cave in Vietnam is so big it has its own river, jungle and climate. Australian photographer John Spies, 59, spent a week photographing the natural wonder of the cave system. "With ceilings towering over 200 metres 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.

"The entrance 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 80m down a steep wall with the use of harnesses and ropes. The huge temperature difference also creates moving clouds of mist, especially near the two karst windows, which gives the cave its magically surreal atmosphere."

"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," said John.