If you get a very powerful lightning in the Earth's surface in the soil column of vitrified SiO2 formed hollow branched tubes (in fact - the glass) with smooth or covered with small bubbles glass surface. Sometimes the individual drops are formed. The appearance of the glass tube due to the fact that the grains are always between air and moisture. The electric current in a fraction of seconds lightning heats the air and water vapor to enormous temperatures (melting point 1700 ° C sand), causing explosive growth in air pressure between the grains and its extension. Expanding air forms a cylindrical cavity within the molten sand. Followed by rapid cooling fixes fulgurite - a glass tube in the sand.
In the photo you see the biggest ever excavated fulgurite. Dig it up in a place called Camp Blanding (US military training camp). And it came to light, we can say, artificially: scientists from the University of Florida conducted his research there on lightning and, in fact, know where to dig.