The names of _stagnum_ or _stagna_, and _naumachia_, are evidently used indifferently by the classical authors. It has been already mentioned that in the description of the far-famed palace of Nero, reaching from the Esquiline to the Palatine Hill, Suetonius also speaks of it having “a lake (_stagnum_) like a sea surrounded by buildings, after the fashion of cities[77].” This could only apply to the Colosseum, and from this it would appear that in the time of Nero the surface could be flooded when required for theatrical display. Probably in two parts, divided by the great central passage or the gulf, and these two parts were called the _stagna_. It must always be remembered that the one object of the whole building was a theatre for the amusement of the people, very much like the Crystal Palace for London. The probability is that some of the walls of the buildings of Nero round his _stagna_ were used as part of the lower galleries of the Colosseum; these walls and arches are a mixture of stone and brick, and some of the brickwork has quite the character of the time of Nero, so well known to Roman antiquaries as the finest brickwork in the world[78]. The excavations of 1874 and 1875 have confirmed this opinion; there is a series of arches over the entrance to the dens for wild beasts, under the _podium_, which are distinctly of the well-known brickwork of Nero. Some of these walls of the substructure are earlier than the time of Nero, others are later, with large repairs of the fifth and sixth centuries. The great external corridors are entirely built of stone, and are evidently added on to the galleries; there is a straight joint from top to bottom in all parts, and no junction with ties anywhere in the original construction. The construction of the walls of the Colosseum in the interior shews such evident patchwork of different periods, that it is impossible to believe they were all built within ten years, as is commonly said.