Worried about sieges from the frequent hostile raids on the city, the Emperor Justinian ordered hundreds of cisterns to be built. Known as Yerebatan Sarayı, or the Sunken Palace, this is the only one open to the public, and it dates from the late 6th century. Located in the heart of the old town, within reach of the Topkapı Palace and other sites, it's easy to spend an hour escaping the sun in this majestic place.
Once you descend the 52 stone steps (paying careful attention to the caution signs), you enter a magnificent vaulted space. Anyone with claustrophobia would not feel hemmed in. Still water stretches eerily before you, with occasional lights sparkling in its surface and help you find your way around. Originally water would have filled the space, but now just a few feet of it lie on the bottom, allowing visitors to make their way around on walkways.
At this 'start' end there's even a small stage built over the water, where occasional performances take place. I can only imagine how strangely atmospheric those must be.
The columns are a glorious mismatch of styles and sizes. Left-overs from other building projects and rejects from major developments such as the Hagia Sophia, any column would do, however plain or patterned. All 336 of them have been built up to be exactly 9m high, so that the cistern has a consistent roof height.
At the far end you find Medusa, buried in the water. Two columns have bases made out of statues depicting her. Most peculiarly, the blocks put her head on its side, or even upside down. One wonders what on earth the builders were doing, drowning such an important figure, but when you remember that this cistern was constructed under Christian rule, it makes more sense. The pagan building materials were too valuable to be thrown away, so were reused in a way to undermine the superstitious power still attributed to them. The pagan influences were supposedly negated by being inverted like this, making the stone fit for purpose once more. That's the theory, anyway, but you're free to make your own guesses, prosaic or poetic; there is a sense of magic about this underground cavern which would easily inspire imaginative responses to its architecture.
Water drips occasionally from the ceiling, reminding you that this water storage area is still fulfilling its function as well as providing a tourist attraction. This can make some areas a little slippery, and although there are warning signs, the location of the drips is too unpredictable for them to be fully accurate. Wearing sensible shoes and walking with caution is advised.
There's a small gift shop on the way out, but thankfully visitors are not pressurised into parting with their money on any grand scale. It's hard to take photographs down there because of the lack of light, so postcards are really useful. If you do want to take your own pictures, try taking them without flash, but at the same time as someone with flash does (from another angle). Their flash allows enough light for your camera to take in the view, without reflection on the water or over-exposure. It's fiddly, but worth the effort.
The Basilica Cistern has even found fame in a James Bond film. In 'From Russia with Love' (1963) Bond makes an implausible getaway across the waters, straight from the Grand Bazaar, which in reality is some distance away.
The site's main website is in Turkish and lacks a clear English translation. You can find out more by searching online for other websites which cover it.
We still don't know the location of all the cisterns, but know that there are lots more of these archaeological treasure troves to be discovered. Rated by the Guardian as one of the top ten sights to visit in Istanbul, this unexpected, hidden place really does deserve a visit.