What does lightning do when it hits the ground? © NOAA

Ever wonder what happens to lightning when it strikes the ground? You might think that it would burn the ground on contact, or just disappear. In fact, lightning often creates what is called a fulgurite, from the Latin fulgur meaning "lightning". Fulgurites are structures made of sand grains that melt and fuse when lightning passes through sand or sandy soil. The course of the bolt through the ground is sometimes preserved as a delicate "tree" of hollow glass tubes. Sometimes whimsically called "fossil lightning," these natural sculptures retain the shape and path of a lightning strike after impact.

How can you find a fulgurite? "All you have to do is go to any beach and start digging," says University of Florida professor Martin A. Uman, one of the world's leading lightning authorities. In 1996, he and his lightning research team were hunting for fulgurites in Florida and unearthed the longest and largest fulgurite ever found. It has three branches — the largest one is over 16 feet long! Excavating the fragile glass structures requires an experienced hand and the delicate touch of a paleontologist. "We found out there's very little done in terms of scientific excavation of fulgurites. There are collectors that go out and shake dirt and pull out as big of pieces as they can," Uman says, "but they don't have the goal to extract them scientifically in a controlled environment for the purpose of exhibiting them as a natural history phenomenon." He hopes that increased scientific interest in fulgurites will turn these remarkable artifacts into more than mere curios.

A fulgurite in the AMNH Earth and Planetary Sciences collection. The shape of a fulgurite reflects the path of the lightning that formed it. © AMNH Department of Earth and Planetary Science, Division of Physical Sciences

This glass tube was loose sand before it was struck by lightning. © AMNH Department of Earth and Planetary Science, Division of Physical Sciences

Uman notes that we understand the behavior and effects of lightning above ground much better than we understand how it behaves beneath Earth's surface. Fulgurites — and what they reveal about what lightning does underground — began as a curiosity for the professor, but they've become an obsession. He reasons that just as we've learned to protect power lines above ground, knowing more about subterranean lightning will help us protect underground power supplies.

© 2001 American Museum of Natural History