Ultrahigh energy cosmic rays come from outside the Milky Way
Huge experiment is trying to track the particles back to their sources
![Pierre Auger Observatory](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2017/09/092017_lg_cosmicrays_feat_free.jpg?fit=860%2C460&ssl=1)
LYING IN WAIT Huge tubs of water (one shown) at the Pierre Auger Observatory in Argentina reveal the tracks left as cosmic particles zip through them.
Steven Saffi