Cast Damascus Steel
“No one has replicated the original method of making Damascus steel because it was cast from wootz, a type of steel originally made in India over two thousand years ago. India began producing wootz well before the birth of Christ, but the weapons and other items made from wootz became truly popular in the 3rd and 4th century as trade items sold in the city of Damascus, in what is modern Syria”
https://www.thoughtco.com/damascus-steel-facts-608458
I think they finally learned how in 2000. Paul Harvey did a segment on it.