The largest natural arch in the world is the Fairy Bridge (Xianren Bridge, 仙人桥) located on the Buliu River in China. The direct translation is immortal people bridge, which is the connection between heaven and earth.
While the bridge has been known about for some time, it was brought to the attention of the western world via NABS (National Arch & Bridge Society) while scrolling Google Earth. An expedition to see it in person was put together in the fall of 2010. A remarkable journal of the trip was published online by Stephen C. Jett which can be read here. Among other interesting events on their trip, they learned of yet another, undiscovered to the western world, enormous natural bridge located in the town they were staying in while on their trip.
Google Map location: The Google Map location satellite view is very shaded and hard to see. Bing Maps doesn’t have the shade but is far too pixelated. Because of this I suggest looking at the location in Google Earth, direct link here: Google Earth location
The natural arch was measured at 400 feet, +/- 15 feet. In 2010, accessing the bridge required either a 3-hour long raft trip from upstream or a much shorter pole boating trip from downstream. Today, there is better road access and amenities at the bridge as it is now a park.
Fairy Bridge Videos
Fairy Bridge Photos
Most of the photos are from Gunter Welz and Ray Millar from the NAB trip in 2010.