Martin Edström and Katja Adolphson are stepping out into the second doline, or collapsed sinkhole, inside Hang Son Doong, the world’s largest cave. The doline is large enough to fit a jungle, its opening being several hundred meters wide. This forest harbours few animals apart from the birds that can come and go, but is rife with insects and bugs. Several new species have been found inside Hang Son Doong, and there are probably more to be expected.