The arena was originally floored with wooden planking, removable for the mock naval battles which were staged here in the early years of the amphitheater’s existence. Since this had been the site of Nero’s artificial pool, flooding must have been comparatively easy. But though slaves fought and killed each other in these naval battles, they were less sanguinary, and therefore less popular, than gladiatorial contests or beast fights, and changing back and forth from murder on water to murder on land was a nuisance, so the naval battles were transferred elsewhere. The area below the arena floor was then filled in with complicated substructures, which finally revealed their secret to Cozzo in 1928. The area under the catwalk in each quadrant contains eight cell-like rooms (A in Fig. 9.5), each big enough to hold a man, and approached by a short corridor. Opening out of each corridor, forward and to the left of a man sitting in the cell, are three adjoining shafts, a small square one (a), a large rectangular one (b), and another square one (c) of medium size. How are these to be explained? Cozzo reasoned that a beast was released from his cage near the center of the substructure, into the corridor (1) shown in Fig. 9.6, with a portcullis (a) at the end of it. The portcullis was raised, and the beast charged into the transverse corridor (2). This was too narrow for him to turn back; he was therefore forced to go forward into the open elevator-cage (3). The attendant in the cell (A in the previous figure) then released a counterweight, whose rope ran in shaft (a) of Fig. 9.5, while the weight itself rose and fell in shaft (c); the elevator-shaft is (b). The elevator door then closed; the elevator rose, activated by the counterweight, to position (4) in Fig. 9.6. The beast emerged into the narrow upper-level corridor (5–6), raced up the ramp (7), and emerged, slavering for fresh meat, through the trapdoor (8) into the arena. [Illustration: FIG. 9.5 Rome, Coliseum, beast elevator. (G. Cozzo, _Ingegneri Romana_, Fig. 170)] [Illustration: FIG. 9.6 Rome, Coliseum, beast elevator, elevation. (Cozzo, _op. cit._, Fig. 175)] [Illustration: FIG. 9.7 Rome, Coliseum, model, showing colossal statue of Nero (left center). Arch of Constantine (bottom left), and gladiators’ barracks (right center). (P. Bigot, _Rome Antique_, fac. p. 44) ]