In the consulate of Marcus Licinius and Lucius Calpurnius, the casualties of some great wars were equalled by an unexpected disaster. It began and ended in a moment. A certain Atilius, of the freedman class, who had begun an amphitheatre at Fidena, in order to give a gladiatorial show, failed both to lay the foundation in solid ground and to secure the fastenings of the wooden structure above; the reason being that he had embarked on the enterprise, not from a superabundance of wealth nor to court the favours of his townsmen, but with an eye to sordid gain. The amateurs of such amusements, debarred from their pleasures under the reign of Tiberius, poured to the place, men and women, old and young, the stream swollen because the town lay near. This increased the gravity of the catastrophe, as the unwieldy fabric was packed when it collapsed, breaking inward or sagging outward, and precipitating and burying a vast crowd of human beings, intent on the spectacle or standing around. Those, indeed, whom the first moment of havoc had dashed to death, escaped torture, so far as was possible in such a fate: more to be pitied were those whose mutilated bodies life had not yet abandoned, who by day recognized their wives or their children by sight, and at night by their shrieks and moans. The news brought the absent to the scene — one lamenting a brother, one a kinsman, another his parents. Even those whose friends or relatives had left home for a different reason still felt the alarm, and, as it was not yet known whom the catastrophe had destroyed, the uncertainty gave wider range for fear.
When the fallen materials came to be removed, the watchers rushed to their dead, embracing them, kissing them, not rarely quarrelling over them, in cases where the features had been obliterated but a parity of form or age had led to mistaken identification. Fifty thousand persons were maimed or crushed to death in the disaster; and for the future it was provided by a decree of the senate that no one with a fortune less than four hundred thousand sesterces should present a gladiatorial display, and that no amphitheatre was to be built except on ground of tried solidity. Atilius was driven into banishment. It remains to be said that, on the morrow of the accident, the great houses were thrown open; dressings and doctors were supplied to all comers; and Rome throughout those days, however tragic her aspect, yet offered a parallel to the practice of the ancients, who were accustomed, after a stricken field, to relieve the wounded by their liberality and their care.
--Tacitus,
Annals IV, 62-63.
'Fifty thousand' is best read as 'a hell of a lot', of course, but still.
The collapse of Circus Maximus wooden bleachers (the cheapest seats) that happened during the reign of Antoninus Pius was reported as having caused 1,112 deaths, which seems more probable.