Schwyll Aquifer (pronounced 'Shwill') was historically known as 'the Great Spring of Glamorgan'.
Now functioning as a backup supply, it has a number of associated source protection zones policed by Natural Resources Wales.
Due to the delay between local heavy rain and discoloration of the water at the spring, it is believed that the origination is 20 miles away.
This would locate the main source in the limestone of the southern edge of the Brecon Beacons.
At 400 metres, a large cavern was discovered containing bones which were identified as horse.