When a building collapses
"If there is fear of an attack by enemies, or of a flood or of the collapse of the building where the Mass is being celebrated, the Mass is to be discontinued if it is before the Consecration; if this fear arises after the Consecration, however, the priest may omit everything else and go on at once to the reception of the Sacrament."(Pius V - De defectibus in celebratione Missarum occurrentibus, 32)