The mosque in the cellar of a Byzantine fortress: the Yeraltı Camii in Karaköy, Istanbul

In Karaköy, on the Beyoğlu shore where the ferries unload commuters bound for the Asian side, there is a door your eye easily passes over. No dome, no courtyard, no ablution fountain in sight: only a street-level entrance and a few steps that go down. Below opens the Yeraltı Camii, the "underground mosque", also called Kurşunlu Mahzen, the "warehouse sealed with lead". It is one of the few mosques in Istanbul carved entirely underground, and the contrast with the light and traffic of the quay, a few steps away, is sharp.

From fortress to mosque

The space was originally the cellar of a Byzantine fortress. A map by Cristoforo Buondelmonti from the 1420s marks a "Kastellion" at this spot, and tradition connects it to the great chain that in Byzantine times was stretched across the Golden Horn to bar the entrance to naval attacks: it is said that one of its ends was anchored right here, on the Galata side. After the conquest of 1453 the cellar served as a munitions store and a cistern. The transformation into a mosque is dated and documented: it was the grand vizier Köse Bahir Mustafa Pasha who converted it in 1753. The Istanbul earthquake of 1754 damaged the structure, which Sultan Mahmud I had rebuilt; the work was completed in 1756.

The interior

Inside, the effect is that of a low hypostyle hall in half-light: a forest of squat, closely spaced pillars holding up flattened vaults, lit by artificial lights and a few openings. The ceiling is near your head, the air is still and damp as you would expect from a space below sea level. Along the perimeter are three cenotaphs, added in 1753 after a dervish of the Naqshbandi brotherhood said he had received a revelation about them in a dream. They are attributed to Arab soldiers whom local tradition holds were captured during the ancient Muslim sieges of Constantinople and killed here. The names inscribed — among them that of Amr ibn al-As — are in fact disputed by historians, since several of these figures died elsewhere, in Egypt or Arabia. The wooden tombs are protected by iron grilles with coloured glass and remain destinations of devotional visits.

Visiting the mosque

It is a mosque in use, not a museum: entry is free of charge, but the usual rules for any active place of worship apply. It is best to avoid the times of the five daily prayers and of the Friday noon prayer, to remove your shoes at the entrance, and to cover shoulders and legs; for women a headscarf is advisable. The visit is short, ten or fifteen minutes, but it is precisely the mismatch between the anonymity of the door and what you find on the way down that makes it memorable.

How to get there

Getting there is simple. The reference point is the Karaköy ferry landing, on the Golden Horn: the mosque is about a minute on foot, at the corner between the waterfront and Kemankeş Caddesi. You reach the neighbourhood by the urban ferries (vapur) from Eminönü, Kadıköy or Üsküdar, or by the historic Tünel funicular that connects Karaköy to İstiklal Caddesi. Coordinates: 41.0224628, 28.9767023. It is a few minutes from the Galata Bridge and the Galata Tower, so easy to slot into a walk along the shore.