Scientists in Switzerland have powered up a huge machine and begun what could be the world's most powerful experiment...
It's being carried out in a machine called a Large Hadron Collider (LHC), and could help answer some big questions... like how the universe began.
The machine is inside a 17-mile long tunnel, buried deep underground on the border of France and Switzerland.
It cost five billion pounds, and took 20 years to build...
And contains more than 1,000 huge magnets!
Scientists powered it up for the first time on Wednesday.
And so far they say their tests are going well.
They say the first beam of particles has already travelled the whole way round the pipe...
When the particles are whizzing around fast enough, the scientists will smash them into each other... and see what happens!