“The Romans were incredible engineers. To slow down the flow of the water they deliberately built curved sections. Teams of diggers would start from opposite directions and then meet in the middle. The excavation was done by specialist engineers, but the spoil would have been removed by slaves,” said Mr Diaz, 52, a Spanish member of the group who lives in Rome and led The Telegraph into a long section of aqueduct near the modern-day village of Mandela.