Ogof Daren Cilau

Getting into Ogof Daren Cilau isn't for the faint-hearted. This is a proper underground river cave system near Crickhowell, and you'll need to wade through cold water and crouch through tight passages to see what lies within. The entrance itself sits in a modest hillside - easy enough to find if you know where to look - but once you're inside, the scale of the place becomes clear. The cave extends far deeper than most casual visitors realise, with stalactites hanging from the ceiling and an active stream running through the middle.

Historically significant too. This spot has yielded important archaeological finds, which tells you something about how long people have been drawn to caves like this. The water temperature is genuinely cold year-round, so proper wellies and a waterproof jacket aren't optional extras - they're essential kit.

It's absolutely free to visit, which is rare for something this geologically impressive. Allow a good couple of hours if you want to explore properly rather than just poke your head in. Families with kids who don't mind getting wet will find it genuinely thrilling - there's something primal about scrambling through cave systems that you simply don't get from most attractions.

If you're based at Cwmdu Campsite nearby, this makes an excellent rainy-day outing or a change of pace from the walking trails around Brecon Beacons. Just go prepared, check the weather beforehand, and respect that this is an active cave system - not a polished tourist attraction.