What was the exact location of the sulphur springs, the Tarentum, and the altar of the infernal gods? I have reason to regard the discovery of the Altar of Dis and Proserpina as the most satisfactory I have made, especially because I made it, if I may so express myself, when away from Rome on a long leave of absence. It took place in the winter of 1886-87, during my visit to America. At that time the work of opening and draining the Corso Vittorio Emanuele had just reached a place which was considered _terra incognita_ by the topographers, and indicated by a blank spot in the archæological maps of the city. I mean the district between the Vallicella (la Chiesa Nuova, the Palazzo Cesarini, etc.) and the banks of the Tiber near S. Giovanni dei Fiorentini. The reports spoke vaguely about the discovery of five or six parallel walls, built of blocks of peperino, of marble steps in the centre of this singular monument, of gates with marble posts and architraves, leading to the spaces between the six parallel walls, and finally, of a column with foliage carved upon its surface. On my return to Rome, in the spring of 1887, every trace of the monument had disappeared under the embankment of the Corso Vittorio Emanuele. I questioned foremen and workmen, I consulted the notebooks of the contractors, every day I visited the excavations which were still in progress, on each side of the Corso, for building the Cavalletti and Bassi palaces, and lastly, I examined the "column with foliage carved upon its surface," which in the mean time had been removed to the courtyard of the Palazzo dei Conservatori on the Capitol. This marble fragment, the only one saved from the excavations, gave me the clue to the mystery. It was not a column, it was a _pulvinus_, or volute, of a colossal marble altar, worthy of being compared, in size and perfection of work, with the Altar of Peace discovered under the Palazzo Fiano, with that of the Antonines discovered under the Monte Citorio, and with other such monumental structures. There was then no hesitation in determining the nature of the discoveries made in the Corso Vittorio Emanuele; an altar had been found there, and this altar must have been the one sacred to Dis and Proserpina, as no other is mentioned in history in the northwest section of the Campus Martius.