In 1979, Eavis arrived in Borneo as part of a British expedition designed to study the rainforest and help the recently independent Malaysia understand the newly established Gunung Mulu National Park. Caving was still a relatively young sport, and Eavis and a team of four others were brought on only after expedition leaders realized that there were huge caves among the forest's many treasures.

With their first discovery they set their record for size: It was called Deer Cave, or Gua Rusa, and its entrance was so enormous—nearly 500 feet high—that the sun reached deep inside and fresh air followed, creating a strange and wonderful habitat in the seam between daylight and darkness. A mammoth colony of bats clung to the cavern's roof, while on the floor thick piles of guano teemed with cockroaches, crabs, worms, and hosts of specialized microbes.