Before humans settled the vast wetland that would one day become Mexico City, the axolotl was an unassuming, if peculiar, species. Unlike most amphibians, the mud-colored salamander retained its tadpole form into adulthood, breathed through feathery gills jutting out of the sides of its head, and lived only in a handful of lakes in central Mexico. Then the Aztecs arrived and decided to turn the axolotl’s home into a city. These newcomers quickly built a maze of artificial islands, slicing up the lakes into a sophisticated network of canals. Axolotls, which thrived along muddy, vegetation-rich banks, saw their habitat increase exponentially. Soon, the strange salamanders were everywhere. An axolotl-shaped god even joined the Aztec pantheon.Six hundred years later, Mexico City’s canals are nearly gone, taking the wild axolotl with them. The remaining canals, located in the southern district of Xochimilco, are polluted and overrun with invasive carp and tilapia, which eat axolotl eggs and devour aquatic plants, destroying the banks where the salamanders once thrived. For the past 3 months, scientists from the National Autonomous University of Mexico (UNAM) have scoured the canals in search of wild axolotls. A similar survey in 1998 counted 6000 axolotls per square kilometer. This year, the scientists captured only one and spotted just six more. Science joined them for a day on the axolotl’s trail.