Dan yr Ogof, NationalShowcaves Centre for Wales

The brothers Edwin and Jeff Morgan of Abercrave found the caves by candlelight in 1912, crossing an underground lake by coracle — which is how Coracle Pool got its name — and the system takes its name from their farm below the cave mouth. Seventeen kilometres have been mapped since, making it the largest show cave complex in western Europe. A 2005 Radio Times poll put it first among Britain's natural wonders. The site is a National Nature Reserve, and alongside the three show caves there is a dinosaur park, an Iron Age farm, a museum and a shire horse centre.