The ancient tech myth I see the most is the idea that the recipe for "Roman concrete" was lost, and somehow modern engineers can't figure out how to make a superior mix.
Certainly, Roman engineers built incredible unreinforced concrete structures. But this was accomplished through structural engineering techniques designed to keep the concrete compressed (e.g. arches, domes). Modern structures like elevated highways and skyscrapers would be impossible to build this way, and require steel reinforcement.
While the mix the Romans used was slightly different (apparently it contained a bit of volcanic ash and used less water), modern engineers deliberately choose a different mix based on the structure. E.g. larger structures with natural reinformcement, like dams, will tend to use a mix closer to what the Romans used.
Certainly, Roman engineers built incredible unreinforced concrete structures. But this was accomplished through structural engineering techniques designed to keep the concrete compressed (e.g. arches, domes). Modern structures like elevated highways and skyscrapers would be impossible to build this way, and require steel reinforcement.
While the mix the Romans used was slightly different (apparently it contained a bit of volcanic ash and used less water), modern engineers deliberately choose a different mix based on the structure. E.g. larger structures with natural reinformcement, like dams, will tend to use a mix closer to what the Romans used.