Sell Gill Holes (wet route)

You'll need proper caving gear and a torch to get anywhere near Sell Gill Holes, and that's rather the point. This isn't a stroll-through limestone cavern with handrails and a gift shop - it's a proper underground river passage near Settle where you wade, climb and squeeze through passages as water flows around you. The wet route lives up to its name. You'll emerge soaked, muddy and grinning like an idiot.

It's genuinely spectacular if you're into that sort of thing. The geology is fascinating - you're moving through thousands of years of water-carved rock - but this is absolutely a specialist activity, not a family afternoon out. You need experience, proper equipment and ideally a guide who knows the route. Solo wandering could get you lost or trapped, which rather defeats the holiday spirit.

The nearest proper town is Settle, which sits at the heart of Ribblesdale and puts you in striking distance of some cracking walks and cave systems. If you're camping at Ribblesdale Pods or Philpin Farm Campsite, Sell Gill works as a half-day adventure for the serious caver in your group. The other campsites - The Dalesbridge and Orcaber Farm - give you similar access.

Go in summer when the water levels are lower. Winter floods make it genuinely dangerous. Factor in three to four hours minimum. It's free, but you'll likely want to hire a guide or go with an experienced friend who knows it, which costs. Not for the casual visitor, but if underground river passages are your thing, this is brilliant.