Why are particles from deep space passing through your head right now?

Right now, roughly one muon per second is passing through a volume the size of your head. Muons are the leftovers of cosmic rays: atomic nuclei, mostly protons, that race through space at nearly the speed of light, from our galaxy, from distant galaxies and from the Sun. When one of them slams into the upper atmosphere, it bursts air atoms apart and sets off a shower of new particles. Most of those fizzle out, but muons barely interact with matter, so they rain all the way down, through roofs, through you, even into the ground.

The discovery started with a puzzle: instruments kept detecting ionizing radiation, even inside sealed containers. Everyone expected it to come from rocks in the ground, so it should fade with height. In 1909 Theodor Wulf carried his instrument to the top of the Eiffel Tower and found more radiation, not less. In 1912 the physicist Victor Hess took three instruments up to 5,300 metres in a balloon. The higher he rose, the stronger the radiation. He even flew during a near-total eclipse to check it wasn't the Sun. His conclusion, that penetrating radiation was pouring in from above, won him the 1936 Nobel Prize.

These visitors leave traces on Earth too: they make the carbon-14 used to date ancient objects, and they occasionally flip a bit in a computer's memory.