BASE jumping is like buildering meets skydiving, although you don't have to be in a city to do it. BASE is an acronym that stands for buildings, antenna, span (bridge), Earth, and parachuting off of any of these qualifies as BASE jumping [source: Burnett]. Unlike skydiving, BASE jumps tend to happen from much lower altitudes, which makes deploying the parachute in time trickier and the sport much more dangerous than skydiving.
The sport is illegal in almost all cities, because the jumper risks seriously injuring himself and pedestrians or motorists when he lands. In many cases, BASE jumpers illegally access the high points from which they're jumping by breaking and entering or trespassing. Two BASE jumpers were arrested in St. Petersburg, Russia after jumping off of the Cathedral of the Apostles St. Peter and St. Paul in November 2011 [source: The Telegraph].
Two Nashville men were arrested for felony reckless endangerment after BASE jumping off of the Sheraton hotel building there in early 2012 [source: Parriott]. The men deployed their chutes in time and were headed to their truck when police arrested them.
Since the law on all these urban sports varies by city or country, it's best to check out local laws before venturing out. At least you should know what you're getting into before you leap.