No matter how long you keep staring and guessing, these oddball objects are almost impossible to fathom. Called fulgurites, they are the remarkable and rare result of what can happen when lightning strikes planet Earth.
A fulgurite is formed when a lightning bolt with a minimum temperature of 1,800°C (3,270°F) strikes sand or rock. Heat melts the substance on impact, fusing the grains into natural glass tubes that follow the branching structure of the lightning bolt deep underground. Over time, the sand around the fulgurite shifts, exposing the fragile tube. Most fulgurites are made from sand, reflected in the unusually high number in the Sahara Desert.
The longest fulgurite on record was dug up by researchers from the University of Florida, USA, in 1996. This impressive tube had two branches, the longest of which was about 5 m (16 ft) long.