Description More about this programme: www.bbc.co.uk Iain ventures into the largest cave passage in the world, Hang Son Doong in Vietnam. The cave system was created by water percolating down from the rainforest above and carving the rock. Several kilometres inside the cave Iain arrives at something extraordinary- an entire rainforest. A miniature version of the world above. It is able to grow below the surface because the roof has collapsed and so light can penetrate down.
