When the large furry creature first scuttled into their jungle camp looking for its supper, the scientists assumed it was a cat.
But after several further visits, when it tamely allowed itself to be picked up, they realised they were actually holding a giant rat.
The monster rodent was the most spectacular discovery in a remote area of Indonesia which experts are describing as a "lost world" of hitherto-unknown animals and plants.
At the other end of the size scale was a pygmy possum thought to be one of the world's smallest marsupials - creatures which carry their young in a pouch.
"These are two animals which were totally unknown to science and we're absolutely thrilled to have discovered them," said one of the explorers who ventured into the thick jungles of Indonesia's Papua province. Scroll down for more...
Rats alive! The giant Mallomys rodent from Papua is five times the size of his British cousins
It was in 2005 that the team first visited the Foja Mountains where, it is believed, no modern human had ever stepped.
On that first visit, scientists discovered dozens of new plants, birds, butterflies and frogs. On their second they were excited to find more, along with the giant rat and the pygmy possums.
"It's comforting to know that there's a place on Earth so isolated that it remains the absolute realm of wild nature," said Bruce Beehler, vice-president of the U.S.-based wildlife group Conservation International.
The giant rat is the biggest known in the world by far and weighs 3lb; about five times as much as a typical city rat. It is 2ft long, plus tail, and shows no fear of humans.
As they travelled through the forest, the scientists heard the calls of birds they could not identify and were convinced there were many more creatures yet to be discovered. They are planning a third expedition next year. Scroll down for more...
Miniature: The pygmy possum from Foja is thought to the world's smallest marsupial
Foja has been described as wildlife's last frontier, where exotic creatures live without any threat from mankind, mainly because there are no roads or tracks and the nearest native villages are scores of miles away. Papua has 104 million acres of tropical rainforest and some of the richest biodiversity in the world. But it is under threat from illegal logging and clearing for palm oil plantations.
Giant cat-sized rats discovered in Papuan 'lost world' | the Daily Mail