CHIMALHUACÁN, Mexico — On the flat salt basin that was once the Aztecs’ great Lake Texcoco, Mexico is building its “door to the world,” an enormous airport the government vows will exist in harmony with the environment.

Officials described a terminal design so green that it would be a “global reference” for sustainability, and they pledged to rescue degraded lands surrounding the airport.

But soon after construction started in 2015, the government appeared to turn its back on part of that promise, ceding land designated on project maps for conservation to local officials for development.

And as construction moves ahead on Mexico’s grandest infrastructure project in decades, the much-heralded environmental protection effort is still so devoid of detail, critics say, that it raises questions of credibility and actually obscures the risk of flooding.