Croesor Quarry

Croesor Quarry is a large underground slate mine in north Wales which was served by Croesor Tramway. It has a single adit in the hillside and ceased operation after a hundred years in 1930 having produced 11,000 tonnes of finished slate annually at its peak. Croesor Quarry is notable amongst Welsh slate quarries for being almost entirely underground, with no significant surface workings.

It was rare in its extensive use of forced air ventilation instead of the more common natural ventilation used in most slate mines. It also generated relatively small external waste tips compared with the volume of finished slates produced; this was largely achieved by back-filling underground chambers with waste once they were exhausted.