(CNN) -

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.

Green Bridge of Wales, United Kingdom

One of the most famous spots in Wales, the Green Bridge of Wales and the rocky Pembrokeshire coastline, are part of a national park. Nearby you can also spot the Stack Rocks, known as the Elegug Stacks in Welsh.

Eventually it's expected that the ocean will wear away the Green Bridge and the middle will collapse, turning it into stacks. Visitors who continue along the coast to see Pen-y-Holt Stack should note that it's in a British Army range and must be visited through walks organized by the Pembrokeshire Coast National Park Authority.

Ayres Natural Bridge, Wyoming

Not all the glory of Wyoming is found at its two internationally famous Yellowstone and Grand Teton National Parks in the northwestern part of the state.

Less well-known but worth the visit is the majestic Ayres Natural Bridge in eastern Wyoming, about 40 miles east of Casper. The 50-foot tall, 100-foot-long natural bridge over LaPrele Creek is the star of this 22-acre park.

Natural Bridges National Monument, Utah

As streams cut into the canyon walls and flash floods further weakened them, the three mighty bridges at Natural Bridges National Monument in the southeastern corner of Utah were carved over millions of years.

The area was declared a national monument in 1908, and the three bridges were given the Hopi names "Owachomo," "Kachina" and "Sipapu" in 1909. Further erosion has made the once thick and mighty Owachomo Bridge more delicate. While still massive and strong, Kachina Bridge did lose 4,000 tons of rock in 1992.

Pont d'Arc, France

A natural bridge formed by the Ardeche River in the south of France, the Pont d'Arc has a world-famous neighbor: the Chauvet cave paintings, the world's oldest known such works, which date back 36,000 years.

Discovered in 1994, the site was named a UNESCO World Heritage Site this year. But the region's natural beauty is also a national treasure. Classified as a French national heritage site in 1982, the 54-meter-long (177-foot-long) natural arch is the only French example of a natural bridge spanning a still-flowing river. It's also a gateway to the river canyon.

Natural Bridges State Beach, California

A famous natural bridge anchors this Santa Cruz beach, which is best known for its tide pools -- go exploring at low tide -- and seasonal monarch butterfly residents. Up to 100,000 monarchs typically move into the state beach's Monarch Grove in mid-October, and they depart the following January or February. There's a party in October to welcome them, and seasonal tours are available.

London Bridge, Australia

Australia's Port Campbell National Park is best known for the Twelve Apostles, towering limestone rock stacks carved by the Southern Ocean. The ocean is still working on the rock, with seven of eight formations still standing.

But there's another gem in this park about 275 kilometers west of Melbourne: London Bridge, a natural offshore rock span that partially collapsed in 1990 and became a bridge without a connection. (People standing on the rocks were stranded when the middle collapsed and had to be rescued via helicopter.) It's a good idea to stay on the accessible walking paths and scenic drives along the coast.

Fairy Bridge, China

China has quite a collection of natural bridges, including one that experts say has the longest span of any natural bridge in the world. The Fairy Bridge (Xianren Qiao), in southwestern China's Guangxi Zhuang Autonomous Region near the border with Vietnam, has a 400-foot span.