Urban exploration (often shortened as urbex or UE) is the exploration of man-made structures, usually abandoned ruins or not usually seen components of the man-made environment. Photography and historical interest/documentation are heavily featured in the hobby and, although it may sometimes involve trespass onto private property, this is not always the case and is of innocent intention. Urban exploration is also commonly referred to as infiltration, although some people consider infiltration to be more closely associated with the exploration of active or inhabited sites. It may also be referred to as draining (when exploring drains), urban spelunking, urban rock climbing, urban caving, or building hacking.
The nature of this activity presents various risks, including both physical danger and the possibility of arrest and punishment. Many, but not all, of the activities associated with urban exploration could violate local or regional laws and certain broadly-interpreted anti-terrorism laws or be considered trespassing or invasion of privacy.
The River Fleet is the largest of London's subterranean rivers. Its headwaters are two streams on Hampstead Heath, each of which were dammed into a series of ponds, the Hampstead Ponds and the Highgate Ponds, in the 18th century. At the southern edge of Hampstead Heath these descend underground as sewers and join in Camden Town. The waters flow 4 miles (6.4 km) from the ponds to the River Thames.