A teeny species of frog with a see-through belly has been spotted in Bolivia for the first time in 18 years.

Earlier this month, a team of conservationists found three Bolivian Cochran frogs living in Carrasco National Park, east of the city of Cochabamba, on the eastern slopes of the Andes, reports AFP news agency. The frogs are no larger than a grape, weighing around 70 to 80 grams (2.5-2.8 ounces) and measuring just 19 to 24 millimeters in length.

The trio was found during an expedition to rescue reptiles and amphibians threatened by infrastructure development in Carrasco National Park, a hive of biodiversity that's home to as many as 700 bird and 3,000 plant species, including 50 endemic species of orchid. The recent construction of a 290-megawatt hydroelectric dam in the area could see as much as 500 hectares (1,235 acres) of forest being cleared, a move that will undoubtedly disturb the habitat of many rare species living there. 

The frogs were handed over to experts at the K'ayra amphibian conservation center at the Alcide d'Orbigny Museum in Cochabamba, who hope to breed the rare species as part of a conservation strategy.

“The rediscovery of this species fills us with a ray of hope for the future of the glass frogs – one of the most charismatic amphibians in the world – but also for other species,” members of the team told AFP.

Glass frogs are a family of amphibians known for the translucent skin that can be found on their abdomen. Thanks to this unusual window into the body, it’s sometimes possible to see the frog’s inner organs.

Most species of glass frog can be found in Central America, particularly Costa Rica and Panama, although a number of species are known to occur in South America, in countries such as Venezuela, Colombia, Ecuador, Peru, and Bolivia.

It’s not exactly clear why glass frogs evolved their transparent belly, but one theory says it helps them to stay hidden in their surroundings. If the frog is illuminated from above while sitting on a leaf, as they tend to do a lot, the transparent belly helps to stop their shadow from being too dark to predators below.

As if they couldn’t get much stranger, some species of glass frog are known to have green bones due to the accumulation of a green bile pigment called biliverdin.