The upper part was originally of wood only, and was burned, having been set on fire by lightning. The three lower stories only are of the time of the Flavian emperors; the upper story was rebuilt and added in the third century, and only finished in the time of the Gordiani, as is shown by the coins representing it. The imperial entrance was from the Esquiline side, between the arches Nos. 38, 39, which is without number. Commodus constructed an underground passage from the arena to the Palatine, which has not yet been discovered, his so-called passage (_on the right in entering_) being that by which the dead bodies were carried from the arena. Dion Cassius says: "Upon the last day of the sports his helmet was taken off and fell through the door where the dead used to be carried out." The area, basement, or ground-floor, was flooded for the naval fights. Surrounding this were the dens, in front of which was a channel for fresh water for supplying the animals with drink--a spring still supplies it; about ten feet above was the movable stage, sprinkled with sand for the combats, and hence called the arena. A few feet above the arena was the podium, or seat of the emperor, vestal virgins, &c., protected from the arena by iron bars. Behind the podium was a double portico, which ran round the whole building. Fragments of the marble chimeras, with long wings, that ornamented the seats of the podium have been found. The three successive tiers were called _cavea_. Above these was a tier for the people; above this one for the "gods;" thus making six in all. The amphitheatre seated eighty-seven thousand people, and there was standing room for thirteen thousand more. The walls standing upon the area, composed of tufa, travertine, and brick, old material re-used, were built at a period long after the building was dedicated, when the naval fights being abandoned there was no longer any occasion for a movable stage or arena as before. They contained the machinery for the stage above, and for the lifts or _pegmata_ to send men or beasts from the area to the arena. Probably these are the walls thus alluded to by Dion Cassius: "He [Commodus] divided the theatre into four parts by two partitions that cut through diametrically, and by right angles, to the end that from the galleries that were round about he might with greater ease single out the beasts he aimed at." [Illustration: PLAN OF THE EXCAVATIONS BELOW THE ARENA OF THE COLOSSEUM.]