Scientists at the world's largest particle accelerator have successfully collided beams of protons at the highest energy levels ever seen.
There was cheering in the control room at Cern, the European nuclear research centre in Switzerland, as one of the biggest and most complicated scientific experiments got fully underway.
The experiment is seen as a major breakthrough in efforts to understand the fundamental nature of the universe.
The BBC's science correspondent, Matt McGrath, was there and spoke to Dr Tulika Bose from Boston Univeristy.
Professor Brian Cox is a particle physicist and a member of the High Energy Physics group at the University of Manchester, and he works on the Large Hadron Collider.
Is this a good way to spend $9bn?
First broadcast 30 March 2010