Close-up: Zoobombing in Portland, Oregon
6 November 2011 Last updated at 00:13 GMT
The US city of Portland in Oregon is home to a unique weekly cycling event known as zoobombing.
It attracts an eclectic crowd of cycling enthusiasts who like nothing more than hurtling down a steep hill at high speed, on souped up children's bikes, late at night.
Some of the bikes are left chained up in a pile in the city centre, which has itself become a local landmark. The ride, often repeated several times in an evening, starts with a journey on Portland's light rail system then a hike to the top of a hill, near the city's zoo.
Peter Bowes went along to see the zoobombers in action.
The Close-up series focuses on aspects of life in countries and cities around the world. What may seem ordinary and familiar to the people who live there can be surprising to those who do not.