That security brought in its train prosperity, and even luxury, of which the symbol is the grandiose Roman public baths. Though Agrippa, Nero, Titus, and Trajan all built baths whose sites and plans are known, the most grandiose, and the clearest in plan, are the Baths of Caracalla, begun in A.D. 211. The Baths of Diocletian, built a century later, are equally vast, but their plan has been obscured by the incorporation into their fabric of the church of S. Maria degli Angeli and the Terme Museum. The Baths of Caracalla, known to thousands of visitors as the summer setting for Rome’s outdoor opera, were built on a vast platform, twenty feet high, with an area of 270,000 square feet, greater than that of London’s Houses of Parliament. Excavations in 1938, when the Baths were being prepared for their metamorphosis into an outdoor opera house, revealed in the substructure vaulted service corridors, wide enough for vehicles, widening out at intersections into regular underground public squares, with provisions for rotary traffic. Access to the lower reaches was by stairs let into the central piers of the main building. The principal entrance to the baths was to the north (over the edge of the platform at the top center of the air-photograph, Fig. 12.4). It was flanked by numerous small rooms which in the difficult post-war years housed teeming families of Italy’s homeless. (Their unique opportunity of a summer evening to admire the sleek prosperity of the operagoers recreated the gulf that yawned between haves and have nots in Imperial Rome, and contributed not a little to Italy’s unrest.) The main bath building was set in the northern half of the great open space provided by the platform, and was surrounded with gardens. Facing these on the perimeter was a variety of halls, for lectures, reading, and exercise. Those on the east and west were contained in curved projections (exedras). A part of the western exedra appears in the lower left corner of the air-photograph. Beneath it in a subterranean vault was discovered in 1911 what was at that time the largest Mithraeum (shrine of the Persian god Mithras) in Rome.[E] To the south (lower right on the photograph) was a stadium whose seats were built against the reservoir which supplied the baths: this was fed by a branch from one of the great aqueducts, the Aqua Marcia. [E] In 1958 Dutch archaeologists excavated a larger one under the church of S. Prisca on the Aventine Hill.