The main block of the baths is distinguished for its axial symmetry. The most prominent room was the circular _caldarium_, or hot bath (just to the right of center in the photograph). It is between its main piers that the opera stage is set. Behind it the vast rectangular open space (82 × 170 feet) is most logically interpreted as a grand concourse whence the patrons of the baths (as many as 1600 in peak hours) could move unimpeded to the bathing rooms of their choice. This central room was groin-vaulted in coffered concrete, in three great bays supported by eight piers (Fig. 12.5). The rooms around the central rectangle, with their enormously thick walls, were ingeniously arranged as buttresses to resist the thrust of the colossal vaults. [Illustration: FIG. 12.4 Rome, Baths of Caracalla, air view. (Castagnoli, _Roma antica_, Pl. 35)] [Illustration: FIG. 12.5 Rome, baths of Caracalla, great hall, nineteenth century reconstruction.] The large open spaces at the east and west ends of the main block were exercise-grounds. The exedras adjacent to their inner sides were decorated in the early fourth century with the splendidly satiric mosaics of athletes now in the Lateran Museum. With their broken noses, low foreheads, and cauliflower ears, they are the very type of overspecialized brutal brawn which intellectuals in all ages have delighted to ridicule. The large rectangular area at the rear center was the cold swimming pool, or _frigidarium_; perhaps the rooms on either side were dressing rooms. Below the pavement of the baths the excavators discovered tons of L- or T-shaped iron bolted together in the form of a St. Andrew’s cross. The possible inference is that some part of the baths was roofed with iron girders, designed to support bronze plates ingeniously contrived to reflect sunlight onto the bathers below. (The evidence for the bronze plates and the sunroom is not archaeological but literary, and, chiefly because the literary source had little or no idea what he was talking about, has raised apparently insoluble controversy.)