Clicking on the photo above will show you the opening clip of the 1971 American documentary film On Any Sunday. This clip shows young boys racing their bicycles on dirt tracks, imitating motorcyclists. This scene is thought to have introduced BMX racing to a national level, which until this time was only known locally in southern California.
Watch for muggles after 6PM on Mondays and Wednesdays. But if you look for this cache at those times, you're welcome to be a spectator.