This article presents the preliminary evaluation of the first terrestrial laser scan survey on the subterranean structures of Hagia Sophia, which was built in the Late Antique Era in Istanbul. The project which was carried out in 2020, aimed to study the underground network of Hagia Sophia which consists of ventilation channels, wells, and other spatial units of various sizes, serving various functions (e.g. a passage, hypogeum, and cellar). This project allowed for a visualization of units that have a total length of approximately 936 m. According to our observations, this infrastructure provided water supply, drainage, and air circulation. The cloud point data derived from the scanning provided us with a plan overlapping with the ground floor, and multiple cross-sections which present the relationship of the different layers with each other. To this end, the survey helps understand the different underground layers, as well as their relations with the superstructures. These relations enabled us to discuss on the atriums of the Theodosian Hagia Sophia and Justinian’s Hagia Sophia. Finally, we could determine the points where the debris and mud has blocked or restricted air circulation in the channels.