Ogof y nos hir, Ogof Ffynnon Ddu (OFD) top entrance

The entrance to Ogof Ffynnon Ddu sits in the Brecon Beacons like a mouth frozen mid-yawn - dark, inviting, slightly intimidating. This is one of Britain's longest cave systems, and the top entrance gives you access to passages that stretch deep underground through limestone that's been carved out over millions of years. What you get depends entirely on your nerve and experience level. Casual visitors can potter about the entrance chamber and the first accessible passages, spotting stalactites and the strange formations that form when water drips through rock for long enough. Serious cavers with proper kit and training go considerably deeper - this place has claimed the attention of speleologists for decades. It's free, which is genuinely generous given what's on offer. Allow at least an hour for a proper look round, though you could spend considerably longer if you're the type to sit and stare at mineral formations. The cave stays cool year-round, so bring a jumper even in summer. Go when it's dry - heavy rain can make the entrance chambers slippery and occasionally flood the lower levels. The site works brilliantly alongside the Brecon Beacons Camping & Caravan Park just up the road, or any of the glamping spots dotted about. Families with older children will find it genuinely atmospheric rather than frightening, though younger kids might balk at the low passages. Photographers love it. Geologists absolutely lose their minds. It's the sort of place that reminds you Wales has some proper wild corners.