Page 109 in their '100 Great Adventures' feature. Right at the bottom right:
Tunnel hacking...?71 - Head off tunnel hacking
You might get frazzled by high-voltage cables or freefall to your death, but tunnel hacking is a thrilling thing to do close to home. Join the covens of urban explorers across the UK who illegally venture into abandoned asylums, factories and Tube stations. (www.28dayslater.co.uk)
Try it if: You're good at outwitting dopey security staff.