Don't try to drive over these bridges.
These rocky natural spans were formed over millennia by the flowing waters of a stream or other water source, which slowly eroded away the rock to create the shape of a bridge.
But are they arches or bridges?
The Natural Arch and Bridge Society, whose Indiana Jones-like members go hunting for these rock formations around the world, makes this distinction: A natural arch is made of rock, with a hole formed by natural forces, they say. A natural bridge is a type of arch, where water is the natural force making the hole.
Erosion created these magnificent structures, and erosion will eventually take them down. One such wonder was Aruba's Natural Bridge, which was first formed by pounding surf eroding its coral limestone. The 100-foot-span gave way and collapsed in 2005. (The smaller Baby Bridge is still standing nearby.)
Here are 15 spectacular natural bridges around the world.