It’s likely to be a common question among San Francisco pedestrians today and Saturday: What are those people doing with ropes, descending down the outside of a building 46 stories high?

They’re not international jewel thieves, but merely the hundred-plus rappellers in this year’s City Skyline Challenge – a fundraising event for young adventurers, organized by the non-profit Outward Bound California. SFGATE is among the event sponsors.