There are certainly many periods of knowledge regression in history. Following the decline of the Roman Empire, many technologies were lost for centuries. In so

There are certainly many periods of knowledge regression in history. Following the decline of the Roman Empire, many technologies were lost for centuries. In some cases the knowledge was preserved within one geographic area but lost to another. Even modern technology can be lost. Look inside any company with a lackluster documentation culture to see this in action on a small scale. Even NASA had to spent considerable time re-discovering how the Saturn 5 was put together. Edit - I think I meant this as a reply to the GP, but I'll leave it here.