On the 21st of April, 1917, the embankment of the Rome-Naples railway gave way a short distance outside the Porta Maggiore: the works undertaken to repair the damage led to the discovery of a subterranean building of the greatest interest. One may say, without fear of exaggeration, that this is one of the most important discoveries ever made in Rome, and it raises a formidable number of problems archaeological, historical and artistic. It is impossible in a brief article to describe the building fully or to attempt an interpretation of the stucco reliefs that decorate it, especially as it has not yet been published in detail. I will therefore, in this short study, merely point out some of its most important characteristics and summarize the result of recent studies.