It is an often believed myth that there is technological knowledge that has been lost. The most believable myth relates to hardening copper, giving it properties beyond what is know to modern metallurgy. It would amaze me if modern metallurgists do not know some useful technology known prior to modern times. Speculations such as the above should be considered in view of the following. While archeologists have found many metal objects from long ago, they have found none indicating technology unknown to modern metallurgists. BTW: It is possible that bronze was mistaken for hardened copper by some body who did not know about alloys. The speculation that some ancient civilization had aircraft is ludicrous in the absence of any reliable evidence. Various large scale designs on plains by ancient cultures (The Incas are one example) are sometimes mentioned as evidence that ancient aliens with aircraft once visited or as evidence that some ancient culture had aircraft. A much more plausible speculation is that those cultures made large scale designs which were expected to be viewed by their gods, often believed to inhabit some realm in heavens above. The most believable myth that might be valid is that Archimedes made a parabolic mirror & used it to focus the sun’s rays on enemy ships, setting them on fire. It is plausible, but unlikely to have actually occurred. It is much more likely that he developed a catapult capable of hurling flaming items at the ships & the mirror technique was a myth started after his time. While it is probably true that he experimented with parabolic (or spherical) mirrors to focus the sun’s rays on a small scale, it is unlikely that he was able to use such technology to set fire to enemy ships. Note that sailors of that era knew how to put small fires out with water & it is extremely unlikely that Archimedes was able to create large fires from a distance.