Medusa in the Cistern, Istanbul

"Did you go to the Cistern?"
Sarah and Andrew both put this on the must do list for our Istanbul visit and was the question they both asked in their first emails to us.
Yes we went, gazed and were amazed.
The Basilica Cistern built in 532, a subterranean structure built as a water reservoir. It held 80,000 cubic metres of water, supported by 336 columns, the roof measures 65m by 143m. Once again the question comes to mind, how did they do it? Not sure I would like to be down there in an earthquake but it has survived many in its many centuries of existance.