&gt; It&#x27;s hard for me to imagine a pivotal art or technology being lost like that, but I&#x27;m willing to bet its not at all uncommon if we look across history.<p>This has happened quite a bit throughout history, actually. Complete social collapse, as has happened many times throughout history, can have a really bad effect on technological innovation. Focusing specifically on the Roman empire (because that&#x27;s what I&#x27;m more familiar with), here are three off the top of my head:<p>* The various methods to making various different kinds of concrete have been lost and rediscovered several times [0]. Early concrete-like materials were used as far back as 6500BC by the Nabataeans. In both Roman and Egyptian times it was re-discovered that adding volcanic ash to the mix allowed it to set underwater. The Roman methods for making concrete were lost again following the collapse of the Western empire and were not rediscovered in Europe until the 1400s.<p>* Roman roads were extremely well engineered [1], to the point that some are still around today. While this wasn&#x27;t &quot;lost&quot; per se (we could always go dig up a Roman road and see how it was made), the Roman road system in Europe deteriorated because of lack of resources and skills to maintain them. [2]<p>* The Romans made extensive use of plumbing [3], but much of that was again lost in Europe after the fall of the Western empire. The Roman Empire had indoor plumbing, meaning a system of aqueducts and pipes that terminated in homes (at least of wealthy people) and at public wells and fountains for people to use. [4] Lead pipe was used (okay, so maybe that part wasn&#x27;t so great), and there were sewer systems [5]. Even the word &quot;plumbing&quot; derives from the Latin word <i>plumbum</i> for lead. Following the fall of the Roman Empire, plumbing development virtually ceased for centuries except for isolated cases of plumbing installed in palaces and castles.<p>These three things are so integral to the modern world we probably don&#x27;t even think about them much, just assume they were always there. But not too long ago (at least on the scale of human civilization), they weren&#x27;t.<p>TL;DR the Romans were awesome engineers.<p>[0] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Concrete#History" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Concrete#History</a><p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Roman_roads#Construction_and_engineering" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Roman_roads#Construction_and_e...</a><p>[2] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;History_of_road_transport" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;History_of_road_transport</a><p>[3] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Sanitation_in_ancient_Rome" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Sanitation_in_ancient_Rome</a><p>[4] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;History_of_water_supply_and_sanitation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;History_of_water_supply_and_sa...</a><p>[5] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Tap_water#Pipe_materials" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Tap_water#Pipe_materials</a>