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Comment by aaronax | Hacker News Reader

Only three real examples on that page.

Petrification: no real loss to society but I will concede that this appears to be an example of actual loss of "progress". Very few people are saying "oh diety, if only I could preserve this animal with perfect color and texture (but discarding all other characteristics).

Greek fire: the history of when this knowledge was lost is unclear. Also it was intentionally kept secret. Certainly we have functionally similar tech now. I'll give 50% credit.

Panjagan: We don't even know if this was a weapon or a technique? Everything about this is incredibly vague.

In summary, this has happened approximately 1.5 times in the past. Not "quite often".

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That list doesn't seem exhaustive in the least. A modern example is Fogbank, a classified nuclear weapons material that we recently had to reverse engineer to successfully rebuild certain warheads.

https://en.wikipedia.org/wiki/Fogbank

Edit: I see someone beat me to the punch with Fogbank. For another example, look to the F1 rocket engine that took us to the moon. Despite having the actual engineering documents, we just don't have the manufacturing skill to rebuild one: https://apollo11space.com/why-cant-we-remake-the-rocketdyne-...

There is another one where they had to recall retirees to restart Stinger Missile production.

The F1 is particularly interesting, because if we'd undergone a program in 1985, we probably could have restarted production, maybe even 1995 - but every decade another n% of the engineering knowhow sluffed off into the permanent dirt nap until there wasnt enough left.

Another recent example was when Jay Leno had a new heat exchanger made for his Chrysler Turbine car, and they were able to call some retirees in to help make a new one.

And that's why the US is constantly at war in one theatre or the other. It's incredibly expensive to keep production lines going - but it's vital because if you don't, you can't scale production up at a moment's notice.

Us Europeans learned that lesson the hard way three years ago and we haven't made much progress ever since the first Russian boot set foot on Ukrainian soil. 100k artillery shells don't sound like much of a thing... but apparently it is.

No, but it is why we keep making tanks, even though the odds of a land war needing large number of tanks is tiny.

Fogbank was a famous case where we forgot how to make it: https://en.m.wikipedia.org/wiki/Fogbank

Similarly expertise in plutonium manufacturing has been lost, and now that countries have run out they are re-learning how they did it 40 years ago.

There's also the still open question on how the pyramids of Egypt were built, and AFAIK NASA lost the knowledge how to build some stuff as well. And the recipe for Roman concrete was also long lost, with researchers being able to reverse-engineer what it likely was only a few years ago [1].

[1] https://news.mit.edu/2023/roman-concrete-durability-lime-cas...

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