I had read previously that Bl Pius IX had miraculously escaped injury whilst visiting the Basilica, and a little digging turned up the following article from the New York Times of 13 April 1905:
To Canonise Pius IXOn either side of the picture are lists of those who survived the incident. To the left are the various dignitaries who escaped, and to the right is a list of seminarians from the Propaganda College who survived, including Burtsell and Rubian. It would be interesting to establish whether the figures in the painting true to life. Bl Pius IX is, of course, clearly recognizable and I suspect that at least the senior dignitaries portrayed are intended to be realistic. If you look at the figure of the Cardinal who is lying underneath a fallen beam in the bottom left of the picture, you will see that he bears a more than passing resemblance to Cardinal Antonelli who was certainly present.
Pope Receives Surviving Witnesses of Supposed Miracle of 1855.
ROME, April 12. -- an interesting ceremony took place this morning in the Basilica of St Agnes, 2 miles outside of Rome. The building stands over the catacombs, where, among others the body of St Agnes is buried.
While Pius IX on April 12, 1855 was receiving the College of the Propaganda in the Basilica the floor gave way and all present were precipitated into the catacombs, 20 feet below. Nobody was injured, and this, by some persons, was considered a miracle.
The only survivors of the accident the Rev. Dr. Richard L. Burtsell of Rondout, N.Y., and Archbishop Rubian, the resident representative of the Armenians in Rome. In the Basilica this morning Dr. Burtsell celebrated high mass and Archbishop Rubian intoned the Te Deum and bestowed the benediction on the members of the College of the Propaganda.
The Pope later in the day received Dr. Burtsell and Archbishop Rubian. The Pontiff took the occasion to speak of Pius IX. He says that many persons were urging him to begin the informative process towards his canonisation.
"Miracle of the Basilica of St Agnes," the Pope continued, "is one of the events which will be brought forward to establish the fact that Pius IX performs miracles. It is a good thing that there are living witnesses to give evidence."
Edited to add:
I forgot to mention that there's another interpretation of what happened to Bl Pius IX. Some superstitious sorts believed that he had the 'evil eye' - not that he himself was evil or malevolent, but that he was an involuntary bringer of ill-fortune. This book explains:
Ask a Roman about the late Pope's evil eye reputation, and he will answer: "They said so, and it seems really to be true. If he had not the jettatura, it is very odd that everything he blessed made fiasco. We all did very well in the campaign against the Austrians in '48. We were winning battle after battle, and all was gaiety and hope, when suddenly he blessed the cause, and everything went to the bad at once. Nothing succeeds with anybody or anything when he wishes well to them. When he went to S. Agnese to hold a great festival, down went the floor, and the people were all smashed together. Then he visited the column to the Madonna in the Piazza di Spagna, and blessed it and the workmen; of course one fell from the scaffold the same day and killed himself. He arranged to meet the King of Naples at Porto d'Anzio, when up came a violent gale, and a storm that lasted a week; another arrangement was made, and then came the fracas about the ex-queen of Spain.
"Again, Lord C------ came in from Albano, being rather unwell; the Pope sent him his blessing, when, pop! he died right off in a twinkling. There was nothing so fatal as his blessing. I do not wonder the workmen at the column in the Piazza di Spagna refused to work in raising it unless the Pope stayed away!"