The Colosseum originally had a retractable roof! This marvel, called velarium, covered about 2/3 of the stadium, including all the spectators. The way it was designed, it created an updraft for ventilation, resulting in what is described as a “cool breeze.” We aren’t actually sure how it was held up. It’s still a matter of conjecture.
Ancient Roman watermill at Barbegal, France. The watermill (a water powered grain mill) was a technological breakthrough, representing the one first innovations to replace human labor. This site, dating to the 2nd century CE, was comprised of multiple mill complex with sixteen overshot wheels on two mill-races, fed by aqueduct.