It should be noted that any list of the wonders of Britain constructed now could do worse than include the cave Porth yr Ogof (location) and its associated waterfalls. The cave swallows the Afon Mellte in one of the largest cave portals in the country. The river then flows in a broad canal a few hundred metres underground before exiting (and occasionally fountaining) into a deep and notoriously deadly cleft. The waterfalls which join the Mellte downstream include Sgwd Y Eira, a large fall which you can walk behind. You can find details of the cave here and the falls here, along with an engraving of the cave portal by Henry Gastineau (1829) which does it justice here. Both the cave entrance and falls are worth a visit.