Seven kilometers southwest of Antigua along the Escuintla road is Ciudad Vieja (Old City), near the site of the first capital of the Captaincy General of Guatemala. Founded in 1527, it met its demise 14 years later when Volcán Agua let loose a flood of water that had been penned up in its crater. The water deluged the town with tons of rock and mud, leaving only the ruins of La Concepción church.
The actual site of the former capital is a bit to the east at San Miguel Escobar; Ciudad Vieja was populated by survivors of the flood. The pretty church on the main square has an impressive stuccowork facade, though it's about two centuries newer than the plaque by the door boasts it to be.