'Sometimes on this trip it seemed like everything we were looking at was new'

The jungle crater of Mount Bosavi in Papua New Guinea teems with so much life that it took just 30 seconds for the scientists from the BBC Natural History Unit to discover a completely new species, and even then they almost squashed it.

Dr George McGavin and his team of biologists stepped from a helicopter on to the mist-shrouded rim of the Papua New Guinea volcano and spotted a frog hitherto unknown to science. It was the kind of find that takes a lifetime for most biologists and was a taste of surprises to come once they had descended into the lost world of the extinct volcano.

"It was mind-blowing," said McGavin. "Allen Allison, a specialist in amphibians from the Bishop Museum in Hawaii, said, 'I think that's one over there by your foot.' I nearly trod on it."

By the time the biologists had reemerged from the crater, 16 more frogs, including one with fangs, had been catalogued for the first time. They saw vegetation laden with so much slimy green frogspawn it seemed to be "dripping with life", never-before-seen giant rats, a tree-dwelling marsupial that looked like a bear and several strange new fish.

It was an adventure that could have been scripted by Arthur Conan Doyle, whose novel The Lost World chronicled the discovery of long-lost species including dinosaurs on an Amazonian plateau. Equally strange, but this time true, the jungle within the 1,000-metre high crater walls of Bosavi revealed stick insects the length of a human forearm, butterflies the size of a paperback book, a tree kangaroo and a fat-lipped tentacled fish that looked like it had swallowed an octopus.

An extremely hairy caterpillar is now awaiting cataloguing in Oxford, where the team has the enviable task of assigning names to its finds. "We might name it Healeyi after Denis Healey's eyebrows," joked McGavin.

The most exciting discovery came soon after the team climbed into the crater, when they came across a giant but friendly vegetarian rat, which may turn out to be the largest in the world.

"This rat was incredibly tame," said McGavin, the head scientist of the BBC Natural History Unit. "It just sat next to me nibbling on a piece of leaf. It won't have seen a human before. The crater of Mount Bosavi really is the lost world."

The Bosavi silky cuscus, a marsupial that lives up trees and feeds on fruits and leaves, climbed on to the shoulder of Steve Backshall, a climber and naturalist.

"I can't begin to describe how it feels to have an animal in my hands that in all probability has never before been seen by science," he said. "Most biologists would consider it a great achievement to name one new species but at some points on this trip it seemed like everything we were looking at was new. The end of every day was like a massive party. It was very special."

Working with the help of local trackers, the team descended into the volcano in January. "We spent a fortnight in the crater," said Steve Greenwood, the producer who managed the expedition. "On the first day Steve Backshall found a Doria's tree kangaroo, coming close to camp. These are extremely rare creatures. He concentrated on searching for creatures in and around the steep rainforest streams and found several species of frog never seen by science before."

The jungle was so remote the expedition team had to organise the planting of fields of sweet potato and spinach in the jungle six months in advance to provide food. Weeks were spent seeking permissions to cross land owned by local clans in which long discussions in smoky huts had to be translated into the local language, Kasua, which is spoken by fewer than 1,000 people.