Porthgain Quarry produced stone which was crushed before being loaded on boats in the small harbour. At one time an aerial ropeway was used to transport the stone down but tramways also ran. A tunnel was built from the centre of the quarry to allow the tramway to reach right down to the dockside.
The harbour side of the tunnel is now gated and used for storage. On a 2003 visit Sub Brit members were able to pass through the tunnel to the quarry with the assistance of Pembrokeshire National Park.