@chu we've also seen plenty of lost knowledge happen even in our own lifetime, not through any insidious effort to bury it but because moving away from certain technologies has been viewed as progress.

The very large steam engines and hydraulic press cylinders built seventy and eighty years ago are effectively "the lost technology of the ancients."

We know how they work, of course, but can never build any more. The considerable expertise required in order to fabricate castings three and four *stories* high is effectively lost. The people who did that work learned through apprenticeship and experience, and for the most part those tradespeople have retired and passed away without passing on the specific knowledge of how to conduct operations at those scales.

This is, by the way, one of the reasons that the US is able to build and successfully fly very very large rockets: the forgings for those rockets are size-limited by the size of the still-operating Heavy Presses (80000 tonne) - which as mentioned are not possible to reproduce without rediscovering an entire industry of heavy foundry work and logistics.

China has made significant strides in this direction, as has India, and Pakistan has been trying as well. But again, this is not a continuity of knowledge, rather it is an entirely new rediscovery, ab initio.