http://www.newscientist.com/article/dn19430-found-natural-bridges-on-the-moon.html
The ASU-run Lunar Reconnaissance Orbiter Camera has revealed two natural bridges (arches) on the moon, likely formed by impact-generated magma deflating below a hardening crust, according to a story in New Scientist.
The larger bridge is 20m x 7m and the smaller is half that size