Lightning strikes sand and is gone in an instant. What does it leave below the surface?

A solid copy of the strike. In sandy ground the silica-rich minerals melt around the path, and the heat disperses into the cold sand so fast that the structure freezes before it can collapse. The result is a fulgurite: a hollow, branching tube of natural glass, its outside crusted with grains that never fully melted, preserving a three-dimensional cast of part of the underground path.

Lightning is visible for a fraction of a second and gone, and in sandy ground it leaves behind a solid copy of itself.

The current entering the ground heats a narrow path at enormous speed. In a sandy setting the minerals are rich in silica, and that is the compound which melts and becomes glass if it cools fast without crystallizing.

The rapid cooling is half the story. The heat disperses into the cold sand around it within a fraction of a second, so the structure freezes before it can collapse. The result is a hollow, branching tube of glass, its outside crusted with grains that never fully melted.

The shape preserves a three-dimensional cast of part of the underground path, and allows the energy and the temperature to be inferred. Not every strike makes a fulgurite that survives, and the final shape depends on moisture, on the minerals and on the strength of the discharge.