On a low mound in a small rectangular compound on hillside behind houses, 75 yards north of a sharp bend in Ailt-y-Powls.
LOCKED All surface features remain intact with the light green paint flaking badly. The FSM pipe stands on a square concrete plinth and a concrete apron surrounds the ventilation shaft. The hatch is locked June 2018: Concrete block securing the hatch has been removed by someone.
Opened in 1960 and closed in 1991.