The 9th Annual Philly Naked Bike Ride kicked off with a pre-party at Glendinning Rock Garden, a small park just across the Schuylkill River from the Philadelphia Zoo.

PNBR, as it is often referred to, is a "bare as you dare" event: some participants cover up, but others ride completely nude -- well, except for their shoes, which are recommended for safety reasons.

This year's six-mile route -- organizers change it each year -- took the participants past famous Philly landmarks such as the Philadelphia Museum of Art, the Museum of the American Revolution, the Pennsylvania Convention Center, and the Cathedral Basilica of Saints Peter and Paul.

The purpose of the event is to promote the safety concerns of bicyclists and pedestrians as well as body positivity, and recognize the human dependence on oil.

"We advocate for cycling to be a major form of transportation," Maria Serrahima, the lead organizer of the Philly ride, told USA Today .