It is a mystery that continues to baffle modern engineers. Why do 2,000-year-old Roman piers survive to this day, yet modern concrete seawalls embedded with steel crumble within decades?

Even Pliny the Elder, writing in Naturalis Historia in 79AD, noted that concrete structures in ancient harbours, ‘become a single stone mass, impregnable to the waves, and every day stronger,’ despite being constantly battered by seawater.

Now scientists in the US think they have found the answer, and it could finally lead to modern sea defences which withstand time and tide.

They discovered that when saltwater mixes with the volcanic ash and lime used by Roman builders, it leads to the growth of interlocking minerals, which bring a virtually impenetrable cohesion to concrete.

"We're looking at a system that's contrary to everything one would want in cement-based concrete," said Prof Marie Jackson, a geology and geophysics research professor at the University of Utah who led the study.