Burning Man is, of course, possibly the most-publicized, best-attended, least-commercial, and consistently innovative "Gathering of the Tribes" regularly celebrated anywhere on the planet.
The event is currently held on the playa at Black Rock Desert in Nevada, maintained by the Bureau of Land Management.
"Burners" (as participants are often called), are expected to create, participate in, and clean-up after, intense acts of communal artistic expression.
One year, some burners brought rocks.