Cağaloğlu Cistern consists of the remains of a massive wall located a short distance north of the Mese. Part of its east wall can still be seen from the west side of Bab-ı Ali Street near Cağaloğlu Anatolian High School. It dates to the Early Byzantine era, perhaps to the 5th century, and consists of alternate bands of rough mortared rubble and of brickwork with a length of 90 meters. Originally it was suggested it might be the remains of Severan Walls of Byzantium. Excavations in 1968 revealed that the wall went 14 meters deep without reaching the bottom, leading to its identification as the eastern wall of a huge open cistern. The identity of this cistern is unknown, but it might be the Theodosius Cistern, Philoxenos Cistern or the Cisterna Maksima, which are known to have been in the general vicinity.