Roman Concrete

I know you've always wanted to know, so here is how the Romans made that concrete that keeps their buildings standing after 2,000 years:

Modern concrete is a mix of a lime-based cement, water, sand and so-called aggregates such as fine gravel. The formula for Roman concrete also starts with limestone: builders burned it to produce quicklime and then added water to create a paste. Next they mixed in volcanic ash—usually three parts volcanic ash to one part lime, according to the writings of Vitruvius, a first-century B.C. architect and engineer. The volcanic ash reacted with the lime paste to create a durable mortar that was combined with fist-size chunks of bricks or volcanic rocks called tuff, and then packed into place to form structures like walls or vaults.

Roman concrete was not as strong as modern concrete, but it has endured far longer than ours will. Top, the Pantheon, still the world's largest unreinforced concrete dome. Below, the Tempio di Venere e Roma, a temple dedicated to two goddesses and inaugurated by Emperor Hadrian in A.D. 135.