The proper denomination should be “San Giovanni in Fonte”. It was built in the fourth century by Constantine, together with the basilica of Saint John in Lateran, on top of a villa and a thermal building dating back to the second century. Later on it was restored and modified with various operations. The external frieze, decorated with the coat of arms of the Chigi family, is a work by Francesco Borromini (1657). Highly interesting is the entrance with a fragment of Roman lintel. The interior has an octagonal plan. In the center middle, there is an eight porphyry columns ring with Corinthian capitals supporting an octagonal lintel over which there is another order of smaller white marble columns. On the walls there are modern copies of Andrea Sacchi’s  works depicting the stories of Giovanni Battista, under which there are frescoes of Constantine’s stories painted by A. Camassei, G. Gimignani and C. Maratta. In a smaller apse, a  fifth century mosaic is worth to be mentioned. In the middle of a sixteenth century round closure is a font, a green basalt urn with a bronze cover, a work by Ciro Ferri (1634-1689).