"they saw the face of the lake darken and the shape of the city dissolve upon it. They slept among the rocks face up like dead men and in the morning when they rose there was no city and no trees and no lake only a barren dusty plain."
Inferior mirage creating illusion of a lake on the Mojave Desert
Public DomainInferior mirage creating illusion of a lake on the Mojave Desert - Credit: Mila Zinkova

The men have just witnessed an inferior mirage typical of desert regions.

A mirage is an optical phenomenon in which light rays are bent away to produce a displaced image of distant objects or the sky. The inferior (meaning lower) mirage is an image which appears under the real object, so that in desert regions, the sky may be reflected as a bright bluish patch appearing on the ground in the distance. For exhausted travellers in the desert it may appear as a lake of water.