The Large Hadron Collider (or LHC) is being switched off for repairs.
It's the world's biggest experiment - to try to help us understand more about the origins of the Universe.
It exists 100m below ground in a 16-mile-long tunnel on the border between France and Switzerland.
It works by smashing tiny particles together at close to the speed of light. But now, the machine is being closed down until the end of 2014 so it can be repaired to run better.
In 2012 it produced a major breakthrough - by finding the probable explanation for why things physically exist - something scientists have been struggling with for decades.