It breaks my heart that till today we don't know how Inca (or possibly their predecessor civilization) constructed Sacsayhuaman and Machu Picchu. It is still unclear exactly how they achieved those structures. We're talking about 140 tonne rocks being put together to form those structures and assembled without mortar in such a way that one can't slip a piece of paper in between rocks.

When the Conquistadors encountered the structures, they had no idea how they were being made or put together. They pulled them apart and used the pulled apart chunks with mortar to build cathedrals that mostly fell apart every time there was a tectonic movement. So much knowledge in those khipus burnt by religious fanatics.

We use cement concrete which is a massive source of CO2 production. If we could have somehow preserved the knowledge of how those structures were built instead of burning their knowledge, and killing their stone masons and scientists, we could have had a very different modern world today.