Roman builders were masters of concrete. That was how they built enormous structures like the Colosseum. They even had a special type of concrete for harbors, that would form a chemical reaction with sea water, and create an underwater concrete so hard that it's still in use.

Then Rome slowly collapsed, and the secrets of concrete were lost, and weren't rediscovered until centuries later. Scientists just figured out the thing with the sea water concrete a few years ago, but they still don't know the formula.

History is riddled with lost and rediscovered stuff.