Local legend states that a giant golden turtle appeared in the lake and bestowed upon the Vietnamese people a magic sword and victory over Chinese invaders in the 16th century.
Some residents of a village west of Hanoi claimed to have been blessed after catching a glimpse of the species' concave shell as it crested above the surface of their lake.
"This is one of those mythical species that people always talked about, but no one ever saw," said Geoff Hall, zoo general curator.
But the species has suffered due to increased killings for food or to make traditional medicine from the turtles' bones.
Development and pollution also led to loss of nesting habitats along rivers, zoo officials said.
The Cleveland Metroparks Zoo has put more than $275,000 (U.S.) into Asian turtle conservation efforts since 2000 and has supported Hendrie since 2003, officials said.
Hendrie and colleagues from Education for Nature-Vietnam had searched lakes and wetlands along the Red River for three years before hearing about the creature living outside Hanoi.
The newfound turtle remains in the lake, and researchers have notified the Vietnamese government of its existence, Hendrie said.
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