I do not know what it was originally intended for, but they said it was built for a reservoir.

It is situated in the centre of Constantinople.

You go down a flight of stone steps in the middle of a barren place, and there you are.

You are forty feet under ground, and in the midst of a perfect wilderness of tall, slender, granite columns, of Byzantine architecture.

Stand where you would, or change your position as often as you pleased, you were always a centre from which radiated a dozen long archways and colonnades that lost themselves in distance and the sombre twilight of the place.
This old dried-up reservoir is occupied by a few ghostly silk-spinners now, and one of them showed me a cross cut high up in one of the pillars.
I suppose he meant me to understand that the institution was there before the Turkish occupation, and I thought he made a remark to that effect; but he must have had an impediment in his speech, for I did not understand him.
We took off our shoes and went into the marble mausoleum of the Sultan Mahmoud, the neatest piece of architecture, inside, that I have seen lately.