Ruins Terme di Agrippa in Rome

You're standing on Via dell'Arco della Ciambella, a narrow little street right by the Pantheon. Take a good look at the houses around you. Hidden in their walls is a huge curved wall. It's part of the great round hall of the Baths of Agrippa, the first of Rome's great bathhouses. That half circle is about twenty-five metres across, and what you see dates from a rebuild in the fourth century.