Or in romes case, and probably some societies around that time, concrete that we have no idea how to duplicate nor want to.
AFAIK the ingredients for roman concrete are known, they're just comparatively expensive and IIRC lead to something that starts off weaker and more fragile and only gradually grows to be tougher over time because whenever it cracks and water gets in it just does the same reaction that formed the concrete in the first place, effectively gluing itself back together. Great for making something that's still a recognizable structure thousands of years later, less so for getting structural support for a building fast and in large quantities.
AFAIK the ingredients for roman concrete are known, they're just comparatively expensive and IIRC lead to something that starts off weaker and more fragile and only gradually grows to be tougher over time because whenever it cracks and water gets in it just does the same reaction that formed the concrete in the first place, effectively gluing itself back together. Great for making something that's still a recognizable structure thousands of years later, less so for getting structural support for a building fast and in large quantities.