The traffic jam leaving the Burning Man festival has eased up considerably as the exodus from the mud-caked Nevada desert entered a second day following massive rain that left tens of thousands of partygoers stranded there for days. An unusual late-summer storm turned the weeklong fest into a sloppy mess, with tens of thousands stuck in foot-deep mud and with no working toilets. The gathering in the Black Rock Desert north of Reno attracts nearly 80,000 artists, musicians and activists for a mix of wilderness camping and avant-garde performances. More than a half-inch of rain had fallen at the...