U.S. biologists have discovered three new species of toads living in Nevada's Great Basin in an expansive survey of the 190,000 square miles ancient lake bottom.

The discoveries, detailed in a paper recently published in science journal, are extremely rare in the United States.

"We've found the toads in small, wet habitats surrounded by high-desert completely cut off from other populations," Dick Tracy, lead scientist on the project, said in a statement.

"These are absolutely new, true species that have been separated from other populations for 650,000 years," said the renowned biology professor at the University of Nevada, Reno.

Since 1985, only three new frog species have been discovered. And toad species are even more rare, with the last species discovered north of Mexico, the now extinct Wyoming toad, in 1968.

The three new discovered species, the Dixie Valley toad, Railroad Valley toad and Hot Creek toad are not connected geographically. They were found in Tracy's 10-year long survey of the desert-dominated Great Basin.

According to a press release, researchers used 30 "shape" metrics and DNA studies to analyze these toads' characteristics to determine if each were distinguishable from the closely related Western toad, found throughout the Western United States.