There’s one type of steel whose production method have been lost to history but people are still trying to replicate today: Damascus steel.
This is a close up of real Damascus steel blade from 13th Century
This is the modern replica used in making a knife:
Damascus steel is pretty legendary, because the method of making it is lost to history, because of its intricate, beautiful pattern, and also because of its durability. Perhaps more surprisingly, traces of carbon nanotubes have been found in real Damascus blades. Now, this is obviously an unintended byproduct of the manufacturing process but it does explain how the material became so highly regarded to the point of being mythical; it had been said that real Damascus blades could cut through a gun barrel (most likely a myth; the same way that supposedly Japanese WW2 mass-produced katana was said to have cut through hot gun barrels. It’s very unlikely at best).
The term ‘Damascus’ itself is also hotly debated: What constitutes a proper ‘Damascus’?
Interestingly, the Damascus steel is not from Damascus. It is actually thought to be from India, though it became associated with Persia and the Arab world. In fact, it got as far as Russia, where it was called ‘Bulat’. There are plenty of theories how they became called ‘Damascus’, like that Europeans who first encountered it found it in Damascus (back then one of the centers of sword production), or it was a corruption of a renowned Arab swordmaker’s name who made these, and so on. Regardless, people from Damascus do not refer to this steel as ‘Damascus’. The truth of the origin of the name is also likely lost to history.
In the 19th century, ‘Damascus’ was the term for ‘pattern-welded steel’ that was used in making knifes and gun barrels. And then in the 20th century, people started marketing their patterned knifes as ‘Damascus’ to latch on to the fame, adding further to the confusion.
Scientists and engineers have been trying to recreate the material from current samples, but so far they could only come close but not exactly replicate the process. To make them properly, a smith would need the correct raw materials and knowledge of the procedures, both of which probably will never be found again.
HOWEVER, by today’s standards, Damascus isn’t all that special with regards to its strength, the same way that Japanese swords aren’t particularly superior to European or other types of swords. Back then, however, seeing something so beautiful and yet durable must have been nearly magical.