My favourite subterranean attraction is not actually a cave, or a mine, but the Basilica Cistern, or Yerebatan Sarnici, which is the largest of the myriad of cisterns beneath the streets of Istanbul. This 6th century Byzantine underground chamber can hold 80,000 cubic metres of water, although nowadays visitors walk on a raised platform above the shallow water, and watch carp swimming languidly below. The ceiling is supported by hundreds of soaring marble columns nine metres high, two of which have huge Roman blocks with the carved head of Medusa at their base. Eerie and magical, the cistern feels like an underground cathedral.
A great place to visit to escape the midday heat, and there's also a small cafe.
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