1. History and rediscovery

The Basilica Cistern, today also known as the “underground palace” (Yerebatan Saray), is the largest surviving underground water cistern in Istanbul. The cistern is located on the First Hill, southwest of Hagia Sophia, in modern-day Sultanahmet area (Fig. 9). The cistern which measures approximately 141 × 66.5 m in ground plan is covered by brick cross vaults supported by 336 columns, some 8 m high, and has a capacity of 78,000 m³.¹ The water collected in the cistern was delivered via 20 km long aqueducts from the reservoir in the Belgrade forest near the Black Sea.

Originally built under the no longer extant Stoa Basilica, the present structure of the eponymous Basilica Cistern was rebuilt under Emperor Justinian I (r. 527-565) after the Nika riot of 532.² The cistern was used to store water for the Great Palace, surrounding buildings and palaces, and for those in demand for water, especially during summer time. Prokopios, an official historian of Emperor Justinian, records that an aqueduct channeled freshwater to the cistern, but that the cistern also stored water, “which had been wasted because of its abundance during other seasons” for summer.³ The latter provides a good sense of the actual quality of such water for drinking and bathing purposes.

Eventually closed and seemingly forgotten by the Late Byzantine period,⁴ the Basilica Cistern was re-discovered in 1545 by French humanist and antiquarian Pierre Gilles, also known as Petrus Gyllius.⁵ Gilles, who came to the city to gather literary sources and investigate material remains of Constantinopolitan antiquities, learnt from the local populace that they were able to miraculously obtain water and catch fish by lowering buckets in their basement floors.⁶ The stories led Gilles to explore the neighborhood until he got access to a house through the basement of which he discovered a forgotten cistern, and recognized the imperial Basilica cistern. At the time, the cistern was still filled by an ancient pipe that collected water from the Belgrade forest and local wells in the winter time.⁷ The cistern, which later provided water for the Topkapi Saray, was the only ancient water reservoir that remained in use until recent times.⁸ The restorations from the 18^th to the mid-20^th century were crucial for the maintenance of the cistern. The cistern was renovated in 1985, wooden walking platforms were added and, since 1987, it has been open to the public. Today the Basilica Cistern remains one of the most important and the oldest public spaces in the city.

2. Architecture and decoration

The symmetry, sheer grandeur of the hypostyle design and sophisticated hydro-engineering of the Basilica Cistern are exceptional. 336 impressive columns set in 28 rows of 12 columns each,⁹ with centers of the column shafts some 4 meters apart, reveal knowledge of columnar architecture based on modular principles (Fig. 1). The vaulting technique is also exemplary. Groin vaults were constructed not of Roman concrete, but of single layers of brick, slightly pitched, laid across the axis of the vault and embedded in very thick mortar (Fig. 2). This kind of extraordinary light vaulting was later advanced and used in other public buildings and churches, like in the domes and half-domes of Hagia Sophia and the pumpkin domes of Sts. Sergios and Bakchos.¹⁰ Therefore, the modular use of square bays and brick vaulting from the Basilica Cistern has played an important role for the standardization of Late Antique and early Byzantine imperial foundations in Constantinople.