Under the long passage which comes in at the south end is a large drain at a considerably lower depth; there are gratings in the paved floor of the passage above opening into it, which had unfortunately been stopped up in some of the great floods, but was partially cleared out as far as the Meta Sudans in 1875. The paved floor of the passage over the drain under the arena is three feet above the level of the pavement, which is of herring-bone brickwork (_opus spicatum_), and the passage before mentioned goes all round the building nearly under the edge of the _podium_. Modern iron steps have been placed for people to descend to the bottom of the building, and under these is seen the ancient iron grating to prevent anything being carried off by the rush of water[165]. From this it is evident that the great drain was to carry off the water used in the canals for the _naumachia_, when the Emperor “ordered the water to be let off and the boards to be replaced.” There are evident marks of a great flood-gate or sluice drawn up, as a portcullis, at the entrance to this drain. It also appears that the vessels were floated down on the wooden framework on which they were dragged along, now made visible, but it does not appear that they could have been floated up also to the level of the canals. The space between the wooden floor of the _stage_, called the arena, and the original pavement being twenty-one feet, the canals were ten feet deep, and yet room is left for the passages and machinery under them. Possibly, but not probably, the whole central space could be floated, excepting just at the south end, where room was left for the machinery. The vessels were probably never removed from the building, but left under the vaults, and dragged out when required. The tufa walls with the grooves for lifts belong to the earliest part of the building, and must be earlier than the time of Nero[166], as has been shewn; and his _stagnum navale_, or _naumachia_, his _venationes_, or wild-beast hunts, and gymnasium, which are recorded as belonging to his great palace, could have been nowhere else but on this spot.