Lightning struck the Sahara desert about 15,000 years ago, and left a sign of its presence behind. The heat from the lightning reached a little over 1,800C and melted certain elements in the soil until they fused together. This solid form, called a fulgurite, most often shows up in sandy soil, or soil with a lot of quartz. It's not unusual to find fulgurites on beaches, or in deserts. It is unusual to find one that is 15,000 years old.
Scientists can use a technique called thermoluminescence to figure out when these fossils of lightning were created. Fulgurites are crystals with a lot of crags in them. Even seemingly perfect crystals have imperfections. These imperfections trap things. Sometimes those things are electrons. Electrons are careening throughout soil more than we like to think, because there are naturally occurring radioactive elements everywhere. In the dirt, there are certain sources of ionizing radiation - radiation that rips an electron off other molecules and especially water molecules. These electrons get stuck in crystalline structures at a steady rate. When the crystal is heated, it glows, and that glow depends on the number of electrons within it. More electrons mean more glow. More years in the dirt mean more electrons. By observing the thermoluminescence of a fulgurite, scientists can figure out when it was created.
What's more, they can sometimes figure out what environment it was created in. Sometimes fulgurites contain tiny bubbles. Those bubbles trap gas that was frozen in them thousands of years ago, at the time of the fulgurite's creation. The gas contains not just atmospheric molecules, but the carbon that came off the plant life around the fulgurite. The 15,000-year-old fulgurite contained carbon from the shrubs and grasses that once must have covered a much wetter Sahara. All it takes is the fossil of a lightning bolt, and scientist know what the Sahara looked like fifteen millennia ago.