Hagia Sophia Museum is one of Istanbul's most important historical structures. Considering its location in Istanbul and the use of the building through history, visible damage is noted in the structure. To determine whether this damage noted on the exterior of the building is present in the interior, the gallery of Hagia Sophia Museum was scanned with ground penetrating radar. The GPR scanning of the floor of the gallery revealed the presence of weak zones in the floor and varying scales of deformation within the structure. Detailed study of the vaults showed that the geometrical symmetry of the curve of the vault dome was disrupted. The results obtained in this study show that necessary precautions should be urgently completed to prevent damage to Hagia Sophia, especially when the earthquake expected near Istanbul is considered. (C) 2017 Elsevier Ltd. All rights reserved.