Forest in a cave - Hang son doong, Vietnam.

I'm a bit fearful of underground, always liking to be as high up as possible (hence tree house obsession), but if I had to live in a cave I'd want one with a forest - who thought you could have a tree house in a cave?! Hang son doong in Vietnam is the largest cave in the world, with amazing giant caverns you could fit buildings in, but where part of the cave roof fell in many many years ago the light coaxed life out of the previous darkness and now a miniature rainforest thrives. I saw this the other day on the BBC's wonderful "How to grow a planet" documentary. I really recommend seeking this programme out (it might still be on iplayer) as it also had fascinating bits on fossilised forests and tree defences against dinosaurs, as well as a wonderful sequence where the warning communications between plants were filmed (they release an until recently undetectable gas when their leaves are cut to warm others about predatory herbivores). Here's a clip of the cave forest segment.

Here's also a couple of photos courtesy of National Geographic taken by Carsten Peter - there's an article on the caves on their website too that is worth checking out.