Caja de Agua is one of San Luis Potosí's most recognisable small monuments: a handsome stone water structure on the Calzada de Guadalupe. It belonged to the old system that brought and distributed water into the city, but its careful neoclassical design turns infrastructure into civic architecture.
It is best seen as part of a walk rather than as a destination by itself. Follow the calzada, look at the churches and public spaces around it, and use the Caja de Agua as a reminder that Mexican colonial cities were shaped as much by water, roads, and neighbourhood services as by their main plazas.