Good heavens, that was ambitious! Presumably that work was planned to extract the North Vein slate?
I see over the other side of the valley that modern work is also using the the Maenofferen/Llechwedd tips as a source of material for crushing to gravel, whether for infill or decorative purposes I don't know.
It's all too easy to sit here and look at the world from the point of view that it's nice to have this history on our doorstep and that it should be left for us to amble in and enjoy it, but the harsh reality is that unless some older workings are re-used, then the industry cannot survive.
On the up side, new work sometimes reveals things that have been lost for decades due to tipping... 😉
Hello again darkness, my old friend...