Fulgurites, from the Latin word “fulgur” meaning lightning, are shards of melted sand, rock and debris found on beaches where lightning has struck. As the bolt reaches the ground, as much as a hundred million volts are discharged into whatever it strikes, where the bolt itself could reach up to five times the temperature of the Sun’s surface.
If the conditions are right, the bolt could instantaneously vitrify silica-rich sand, which results in the production of thin branches of glassy solids buried in the sandy ground. The branches can reach up to 15m into the ground, but are usually too fragile to remove without breaking. When the sand supporting the brittle structure is removed, the glass is unable to support its own weight and can shatter. This is why most fulgurites are only a few centimetres in length, with longer ones reaching around 75 cm. The longest fulgurite (4.9 m) was found in Florida in 1996.
These structures can also be artificially induced using high voltage rods, as shown in the video: https://www.youtube.com/watch?v=SMlW1APWTVk