Matthew 24:2 - Ellicott's Commentary On The Whole Bible

There shall not be left here one stone upon another. — So Josephus relates that Titus ordered the whole city and the Temple to be dug up, leaving only two or three of the chief towers, so that those who visited it could hardly believe that it had ever been inhabited (Wars, vii. 1). The remains which recent explorations have disinterred belong, all of them, to the substructures of the Temple — its drains, foundations, underground passages, and the like. The words fell on the ears of the disciples, and awed them into silence. It was not till they had crossed the Mount of Olives that even the foremost and most favoured ventured to break it.