The pre-modern/medieval Japanese iron industry was actually quite massive! We went to a museum in Izumo and there was a nice map showing the are in ancient time

The pre-modern/medieval Japanese iron industry was actually quite massive! We went to a museum in Izumo and there was a nice map showing the are in ancient times and now and the difference was pretty stark - there were just swamp where the Izumo city is today and the local lake Shinji was like twice as big as today. All the new land and end of the swamps is apparently the result of hundreds of years of iron ore mining in the nearby mountains. So even with primitive means and shitty ore, if you need the material and go at it for centuries, you can achieve substantial results. Not to mention reclaim some land as a result. :) Also in related (but much more recent development) there were coal mines in Japan mining from under the seabed via tiny islands! Hashima is the most extreme example, basically a piece of barely dry rock that has been converted to a concrete city housing many thousands of workers and their families: https://en.wikipedia.org/wiki/Hashima_Island https://en.wikipedia.org/wiki/Hashima_Island But there were other such mines, some of the local ones even connected to Hashima via the underground works!