The LHC tunnel is 27 kilometres around - longer than most cities. Inside the tunnel, scientists send tiny particles (smaller than atoms) whizzing around the loop. The particles travel almost as fast as light itself - over 1 billion kilometres an hour - and crash into each other on purpose, so the scientists can study what they are made of.
Why do this? Because the smallest things in the universe make up everything bigger - including you, this room and the stars in the sky. By studying tiny particles, scientists can understand how the whole universe works. In 2012, CERN scientists found a tiny particle called the Higgs boson, which helps explain why anything has weight at all.
CERN is also where the World Wide Web was invented. In 1989, a British scientist named Tim Berners-Lee was working there and wanted an easier way to share documents with colleagues. He invented the web so any computer in the world could open the same page. Every website you have ever used began with that idea.
About 17,000 people from over 100 countries work together at CERN. Scientists who would never meet otherwise share offices, swap ideas in the lunch queue and write papers together. It is one of the biggest examples in the world of countries cooperating to do something only humans together could do.