This article discusses a subterranean building, situated north of Hagia Sophia in Constantinople, which was investigated during a recent interdisciplinary survey conducted by Çiǧdem Özkan AygÜn. Although it is generally accepted that the edifice had more than one phase of use, the date of its original construction and utilisation has been problematic since the building is not mentioned in any written sources and was either not included in other archaeological excavations and surveys or not studied intensively. The aim of this paper is to present the underground building in detail and to propose a date for its construction based on the new survey data and on ancient written sources about the church of Hagia Sophia. Archaeological data from previous surveys are also taken into account. The subterranean building's different phases of use are documented, and it is proposed that it was originally used as a reliquary, then later, after a number of alterations, became a cistern.