Denehole

Chalk tunnels dug by hand centuries ago sit under the streets of South London, and Denehole is one of the few you can actually explore. It's not a natural cave - medieval and Roman workers quarried for chalk and flint here, leaving behind a labyrinth of passages that tunnel down into the earth. The entrance is modest, almost easy to miss, but once you're inside it's genuinely eerie. You're walking through grooves worn by people's feet over a thousand years ago.

History buffs will find it absorbing. The passages are cramped in places, which adds to the atmosphere rather than detracting from it. Kids tend to love the novelty of going underground, though under-tens might find some sections claustrophobic - it's not a wide, easy stroll. The whole thing takes maybe 45 minutes to an hour depending on how much you linger and peer into side chambers.

Finding it requires a bit of effort. Access isn't advertised like it is at major attractions, and you'll need to sort out arrangements in advance - it's not a simple "turn up and enter" affair. Worth the faff though if you're into industrial archaeology or just fancy something genuinely different from the usual London attractions.

If you're staying at Abbey Wood Experience Freedom Glamping nearby, it's easily combined with a walk around the local area. The South London setting means it works well as part of a wider exploration of the area rather than as a destination in itself.