Magnetometers come in many shapes and sizes – an ordinary hand-held compass is the simplest – but alkali-vapor magnetometers are extrasensitive devices that measure magnetic fields using light and atoms. They can detect archaeological remains and mineral deposits underground by their faint magnetic signatures, among a host of other scientific applications. Read more at http://newscenter.lbl.gov/feature-stories/2010/09/14/putting-a-spin-on-light-and-atoms/