In this study, the authors have thoroughly documented the various decorations of walls and vaults in the building underneath the Baths of Trajan. This edifice was part of Nero's Domus Aurea, situated in the Augustan Regio III alongside the southern slope of the Colle Oppio in the modern Parco delle Terme di Traiano. This building was a large pavilion within Nero's properties which extended over the Oppius and Esquiline hills. It was built after the Great Fire of July AD 64, which devastated a large part of the centre of Rome and gave the Emperor the occasion to fulfill a long-fostered wish, i.e. that of connecting his properties (forming more or less a series of horti) with the official palace on the Palatine. It has a depth of 55 to 60 metres and its actual length is around twohundred metres. Some hundred and fifty rooms have been preserved. The height of the rooms varies between ten and eleven metres.