“Those secrets may be revealed by muography, a technique that makes use of particles called muons, the heavier cousins of electrons. Muons are created when cosmic rays flying through space collide with air molecules in Earth’s atmosphere. Every square centimetre of our planet is showered with muons at all times. But as they move at near the speed of light and interact weakly with matter, we do not notice them. Compared with imaging techniques such as X-rays, muons are freely available and can be found just about anywhere.”