Fulgurites

When lightning strikes a stretch of sand, most of its enormous energy discharges in a fraction of a second, but it leaves behind something that can last for centuries: a fulgurite. The heat generated as the current forces its way into the ground -- often exceeding 1,800 degrees Celsius -- melts the surrounding sand grains into glass, and the fused tube traces the exact branching path the current took as it fought downward and outward through the sand. The word comes from the Latin "fulgur," meaning lightning, and the tubes themselves are often hollow at the center, since the current's path is narrower than the zone of glass it fuses around it. Fulgurites turn up most often in loose, sandy environments where lightning has an easy path into the ground, which is why deserts such as the Sahara and open sandy beaches are common places to find them. Geologists study fulgurites partly because their branching shape is essentially a fossilized record of a real lightning strike, and partly because the glass composition can sometimes preserve clues about the sand's mineral content or moisture level at the moment of formation. Excavating one intact is difficult, since the glass walls are often thin and brittle, and even a careful dig can leave a fulgurite in pieces rather than as one continuous tube.