At a height of 290 feet and spanning 270 feet, Rainbow Bridge is the world's largest natural bridge and is accessible by way of Lake Powell. The Navajo consider it a sacred place. Visitors cannot walk over it or directly underneath it, although they are allowed to go around it.

From what I've read it seems that the bridge was formed by a stream that once meandered through the landscape. Eventually the stream cut through some sandstone and took a shortcut of sorts, forming the bridge. This is, of course, a grossly oversimplified explanation.