A holy spring, or hagiasma, was a popular pilgrimage site of the Byzantine world. Each hagiasma was associated with a particular saint and viewed as a source of healing. In Constantinople there were three famous hagiasmata located at the monasteries of Blachernai, Hodegon and Zoodochos Pege.
Sacred springs were commonplace in the ancient world. Apparently even the monumental fountains (nymphaea) seem to derive from grotto shrines at springs associated with nymphs. In some instances associated with pagan deities were Christianized, as a saint came to take the place of an earlier divinity. For example, the saints Menodora, Metrodora, and Nymphodora succeeded the ancient nymphs at the famous hot springs of Pythia Therma in Bithynia. There are several traditions explaining the origins of the hagiasma at Pege. One tradition records that Justinian (527-565) came across its spring once when hunting. After being cured there, he ordered a large church to be built on the site of the hagiasma. Another tradition claimed that Leo I (457-474) founded a church here before he was emperor. When he was still merely a soldier, Leo I met a blind man on the road, begging for water. Leo guided him until they reached a marsh filled with mud. Leo tried in vain to find fresh water, until a voice of Theotokos called out and assured him there was water. Theotokos then told him to rub mud over the blind man’s eyes, which restored his sight.
Water was often associated with healing. The hagiasma, along with relics of saints, were viewed as particularly powerful sources of healings. Water was also associated with Byzantine hospitals, as baths as well as cisterns were a common feature of them. The holy springs in Constantinople were also considered to be particularly healthy sources of drinking water, preferable to water kept in the city's cisterns. As the springs at Blachernai and Pege were located beyond the city walls, they were also associated with the relief from the stresses of city life. In some cases, an hagiasma was associated or connected to a louma, or holy bath – as found at Blachernai.
Icons connected to the hagiasmata at Pege, Blachernai and Hodegon later became famous. Early in the Palaiologan era, the epitaph Zoodochos Pege was first applied to the Virgin of the Spring and a new iconography developed, perhaps based on a mosaic in its hagiasma. A mosaic image at Pege empowered the spring and the spring, in turn, empowered the icon of Zoodochos Pege, while the hagiasma at Blachernai and Hodegon were eventually overshadowed by their icons.
The hagiasmata at Pege and Blachernai still exist, though the churches on these sites are more recent constructions. In 1922-1923, excavations at Topkapı uncovered the ruins of several structures, including ruins that often identified as the hagiasma of Hodegon. Another hagiasma was located as the Monastery of Christ Philanthropos, often identified with a substructure within the walls near Topkapı Palace. In 1997-98 an excavation by the Istanbul Archaeological Museums in Sultanahmet discovered an hagiasma in a vaulted chamber under Amiral Tadfil Street. The subterranean chamber had a small pool of water in a niche with a lunette decorated with a fresco of the Theotokos Orans.
Fountains and Water Culture in Byzantium edited by Brooke Shilling
Bildlexikon zur Topographie Istanbuls: Byzantion, Konstantinupolis, Istanbul by W. Müller-Wiener
Pilgrimage in Medieval Asia Minor by Clive Foss