Every few years, out comes a movie about scientists uncovering, in the ruins of a vanished civilization, an "ancient technology too powerful for mankind." Turned out archaeologists already found it. But it didn't quite play out like the horror movies.
In 1961 Scotland, Professor Ian A. Richmond of Oxford University happened on a trove that no Briton was meant to discover. It had been hidden by, arguably, the most powerful and technologically innovative empire the world has ever seen. Forced into retreat by the Celts, a Roman legion had hidden this incalculably valuable technology. They'd gone to great lengths, picking it out of the remains of their destroyed fort, and burying it deep underground, where no one could ever find it. That is, until one archaeology professor, searching for knowledge, uncovered a weapon so powerful it might have changed the face of the Earth.
Sounds like a great start for a horror movie. In these films, unlucky archaeologists or anthropologists are always stumbling upon ancient evils too powerful to be allowed out into the world. Usually they're biological agents that turn people into lizard monsters, or they're star gates that help people travel between dimensions, or they're just weird glowing face-melters. The point is, they are weapons of unimaginable power and the people of their time did everything they could to keep them out of the wrong hands. The artifacts found in Scotland were the same.
The artifacts were nails. They were steel nails, and thus incredibly valuable and dangerous. Steel is an alloy of iron and carbon. Getting one's hands on a great deal of iron is hard enough. Processing it into a material with just the right amount of carbon — over 4% will render the alloy brittle and useless — is another long learning process.
It wasn't quite the uranium enrichment of its time. Making steel could be done by individuals, but it required generations of trial and error. People often didn't even know what it was that made steel strong and brittle. All they could do is keep processing and reprocessing iron, while varying factors like the heat, chemistry, and the oxygen supply of the heating materials.
The eventual success of the project didn't just lead to stronger weapons. It led to the ability to make superfortresses—- like the one that held the 5,000 men in the legion for the better part of a decade. This was a material that could change architecture, transportation, and infrastructure. The fort contained seven tons of nails, which is quite a cache of versatile tech to be left in the hands of the enemy. The legion picked through the ashes of the fortress to find their tech, and buried it.
Until the archaeologists discovered it... and doomed us all.