Two thousand years ago, Roman builders constructed vast sea walls and harbor piers. The concrete they used outlasted the empire — and still holds lessons for modern engineers, scientists say. A bunch of half-sunken structures off the Italian coast might sound less impressive than a gladiatorial colosseum. But underwater, the marvel is in the material. The harbor concrete, a mixture of volcanic ash and quicklime, has withstood the sea for two millennia and counting. What's more, it is stronger than when it was first mixed.