The Gloddfa Ganol slate mine is at the top of the slope.
Slate on the northern side of Snowdonia was extracted from huge quarries, but the geology around Ffestiniog required true, subterranean, mining. Consequently, after Oakeley Quarries (1818-1970), once the largest slate mine in the world, closed, Gloddfa Ganol was able to reopen in 1974 as a slate museum. As at the neighbouring Llechwedd Slate Cavern museum, tourists were able to go underground, which has to be more exciting than visiting an open quarry.
Though Llechwedd remains a thriving attraction (one of the most popular in Wales, I believe), Gloddfa Ganol closed in 1997, as it was considered more lucrative to reopen the site as a working quarry, extracting usable slate from the spoil.