As the cable cars that connect downtown Medellín – Colombia’s second city – to the hillside slums pass overhead, a band of teenage cyclists have gathered at the side of the road. Vallejuelos is a downtrodden neighbourhood, rife with crime and unemployment, but “gravity biking” is helping some kids escape their troubles.

As the name suggests, gravity biking involves steep descents. Enthusiasts strip a typical bike to its frame and build it back up from scratch. To improve descending speed (which can reportedly reach up to 77 mph), riders weld weights to their contraptions. Pedals are superfluous: to climb the hills riders hang on to passing trucks, sometimes using homemade hooks on a line.