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The waters of Xochimilco - shaped by traditional farming practices and flushed with spring water from the mountains - used to teem with these amphibians.

The scientists released their captive-bred animals at two sites - one in Xochimilco and one at a disused quarry that, over decades, has turned into what they called an "artificial wetland".

Xochimilco, a borough of Mexico City, is home to the last remaining canals from the once-extensive lake and waterway system that covered the Valley of Mexico before Spanish colonization.

In the contemporary, helter-skelter sweep of Mexico City, there is one place — in the southern borough of Xochimilco — where a vision of a watery, pre-Columbian capital may still be imagined.

Studies show that Xochimilco’s canals are shrinking and heavily contaminated.