"optically stimulated luminescence" (OSL), a technique often used to date particularly ancient sites. It does this by measuring tiny quantities of light emitted from crystalline material, especially the quartz often found in sand.

The technique uses the quantity of released light to calculate when a grain of sand or other crystalline material was last exposed to sunlight and therefore how long it has been buried for.

Virtually everything on earth is subject to low levels of radiation from naturally occurring radioactive materials. In crystalline material, this radiation produces tiny electrical charges in the form of free electrons.

Some of these electrons get trapped in defects in the crystalline matrix. Normally this trapped energy manages to escape – but when material is buried, the solar light energy which triggers the release of trapped electrons is no longer available. As a result, the electrons stay trapped inside the defects in the crystal until the material is re-exposed to light.