Ogof Ffynnon Du National Nature Reserve

The cave entrance sits dark and uninviting at the bottom of a limestone gorge, water trickling from the mouth like the whole thing is perpetually damp. Ogof Ffynnon Du - Black Spring Cave - is one of Wales's longest cave systems, and walking into it feels properly otherworldly. The reserve protects not just the cave itself but the whole valley around it, where rare plants cling to the cliff faces and the air smells of stone and moss.

You'll need proper gear if you want to go deep inside - this isn't a lit tourist cave with handrails. Most visitors stick to the entrance chamber and the streamway, which is easily done in trainers on a dry day. Cavers and spelunkers absolutely love it; families with young kids will get maybe an hour out of a surface visit before everyone's cold and damp. The surrounding reserve is the real winner though - serious walkers and naturalists will spend half a day picking their way along the gorge, spotting plants you won't see elsewhere.

It's free to visit, though the nearest proper facilities are back toward Swansea, a good 20 minutes away. Go in summer when it's drier. If you're based at Brecon Beacons Camping or any of the glamping spots scattered around Four Falls or Aber, this makes a solid afternoon detour - different enough from the usual Beacons walking routes. Just don't expect theme park accessibility. This is raw Wales.