The remains of an Auxilliary Unit Operational Base, set just below ground in a woodland area, east-north-east of Plas Cilybebyll. Built in areas where a German invasion was expected in the Second World War, the structure consists of a single room, measuring 4.25 meters in length and 3.07 meters in width and 1.95 meters in height, with a curved corrugated sheet steel roof and side walls resting on low sills. The entrance shaft is located at the north-east end of the building.
 

Sources: 

Cadw scheduling description of March 2011
Martin Locock, 'An Auxiliary Unit "Hide" (Operational Base) at Cilybebyll, Neath and Port Talbot, British Resistance Archive
Pontardawe Patrol, British Resistance Archive