Although no coalmining currently takes place on the hill, two collieries were located there in the first half of the twentieth century. The numerous earlier levels had accessed the coal from levels on the east side of the hill. At a much larger scale the Cwm Mawr colliery, based further south in Cwmafon, opened up a No 3 slant around 1920, running into the side of Mynydd-y-Gaer. From 1922 it was owned by Briton Ferry Collieries Ltd, who employed upwards of 100 men at the Cwm Mawr mines for the next 20 years. It closed in 1944, with a short-lived re-opening in 1950.