Their structures are still standing more than 1,500 years after the last centurion snuffed it: now the Romans’ secret of durable marine concrete has finally been cracked. The Roman recipe – a mix of volcanic ash, lime (calcium oxide), seawater and lumps of volcanic rock – held together piers, breakwaters and harbours. Moreover, in contrast to modern materials, the ancient water-based structures became stronger over time.

Scientists say this is the result of seawater reacting with the volcanic material in the cement and creating new minerals that reinforced the concrete. “They spent a tremendous amount of work [on developing] this – they were very, very intelligent people,” said Marie Jackson, a geologist at the University of Utah and co-author of a study into Roman structures. As the authors note, the Romans were aware of the virtues of their concrete, with Pliny the Elder waxing lyrical in his Natural History that it is “impregnable to the waves and every day stronger”.