From CoC

From the Bryn-Seion - Nant-y-Geisiad road, follow the Afon Dulais downstream for about half a mile. The entrance of the mine is a 6 foot high by 6 foot wide opening about 10 feet above the stream. The average cross section is 4 feet high by 6 feet wide and the country rock is a soft slate. The limestone begins about 15 feet below the entrance. Throughout its length, the mine is muddy underfoot and speleothems of iron and calcite are present. The heading ends in a working face, and appears quite sound. There are no remains of surface buildings, and no indication of the mineral being sought

The farmer has since filled in the entrance.

Marked as a shaft on 1st ed. 6inch OS map

Bat roost.