Amphibians are the most threatened animals on the planet, with one third of species at risk of extinction.
Some researchers even say the global population decline is a sign that the world's sixth mass extinction event is under way.
Robin Moore, from Conservation International (CI), is the scientist leading the search for the Amphibian Specialist Group (ASG) of the International Union for the Conservation of Nature (IUCN).
"Scientists are optimistic about the prospect of at least one rediscovery"
Moore told reporters: "This role as the global 'canary in a coalmine' means that the rapid and profound change to the global environment that has taken place over the last 50 years or so – in particular climate change and habitat loss – has had a devastating impact on these incredible creatures."
However, biologists hope they will discover that some of these 40 species are still hanging on.
"Although there is no guarantee of success," Conservation International said in a press release, "scientists are optimistic about the prospect of at least one rediscovery."
The group also compiled a list of the 10 "most wanted" species.
"We're limited by our knowledge of many of these species and whether they even exist - if we don't know whether a species exists, we can't protect it," Moore said.
"So it really is a mission to increase our knowledge of what's out there, what's still alive, so that we can follow up and hopefully do some conservation work on species that are found."