This is the surge pool cut into the opposite hillside of Elidir Fawr. It is a huge water column which drops down to the Hydro Electric Power Station below. When they shut down the turbines the gigantic volume of water that has been flowing into them 'backs up' and the the energy needs releasing. The water column does just that. It is security protected by CCTV & fences as dropping a large object into the column would lead direct to the turbines.
UNESCO World Heritage Site