To the centre of the earth! German photographer Malte Clavin is one of the few who was able to take part in what is probably one of the most adventurous photo tours in the world, the Son Doong expedition. The five-day tour in Phong Nha-Ke Bang National Park first leads through leech-infested jungle and the participants have to ford countless rivers with chest-high water. Roped down into the cave, the tour goes over hundred-metre-high sharp rocks, through underground lakes, caves with their own climate (and cloud formations!) and rare cave forests. Son Doong is the largest cave in the world and was only fully mapped in 2010.