- Location
- Mid-Atlantic
Er, what problem is this a solution to? I mean, I suspect to a large extent this has already happened. Rome did a lot of what would have been incomprehensible megaprojects on the scale of just about any previous civilization other than maybe China. Some of their work was so impressive that medievals actually straight-up forgot it was even possible to do something that big and decided it must have been constructed by wizards or giants or something. They already expanded metalworking demand- and concrete demand, and so on- by a very large factor.
It's very important to realize that Rome was not underdeveloped in any reasonable sense of the word. Rome, like China for most of its history, was very thoroughly developed, up to the limit of the technology. The main reason they didn't have an industrial revolution was that far too many of the underlying social and physical technologies simply didn't exist, and would have to be invented over the course of the next 1500 years or so.
@veekie already sort of covered this, but basically:
Rome fell.
Why did the complexes close? They were overrun by Visigoths or whoever. The invasions tended to kill some of the workforce, and afterwards whichever barbarian king ruled that part of what was once the Empire wouldn't be able to ensure that travel remained safe and practical. Getting new slave laborers for the mines was harder, the usual solution of using convicts instead of slaves didn't work either because the supply of convicts dried up with the fall of the Roman court system. Getting supplies was much harder. Getting the products out of a mine and to market was exponentially harder, and it was this last that was most crippling because it meant there wasn't even any point in keeping the mine running if you couldn't sell its products.
Nor was there much point in running a facility like the mill at Barbegal, which was a major Roman production facility, one of many like it- others were ironworks. These were arguably the greatest industrial facilities prior to the industrial revolution itself, with water-powered machinery that wouldn't have been too far out of place in 1800 as far as I can tell. But without the greater interlocking system of Roman society, sooner or later the know-how to maintain such a mill would be lost, or something important would burn down and there wouldn't be resources or money to replace it. So these facilities collapsed, as did the industrial output behind them.
How did they deal with the loss of metal?
Well, Roman civilization collapsed and people made do with less metal tools and more expensive metal weapons and higher taxation by medieval warlords and aristocrats to support their use of more expensive weapons. And, in short...
Rome fell.
That's a big part of what we mean by the fall of Rome. It wasn't only a political transformation or a decentralization of power, though it was both those things as well. There was a massive economic collapse of goods and services and infrastructure, things which had been sustained by Rome maintaining a quasi-stable social and political order throughout the region, where even if claimants to the imperial throne fought each other, at least there WAS a throne and the wars were being fought by men who aspired to own the countryside they were fighting over, limiting their incentive to pillage.
What replaced Rome, at least in Western Europe, was a less populous and far more fragmented political system, where some important technologies were lost and had to be rederived, where access to literature was reduced, where all manufactured goods were more expensive and often cruder, and where it would be almost a thousand years before people began to once again construct on anything like the scale they had before.
