The Large Hadron Collider (LHC) has been re-started following a programme of repairs to rectify damage caused by an accident last year.
The fault, which caused one tonne of helium to leak into the tunnel that houses the LHC, occurred just nine days after the collider was switched on to much fanfare.
The European Organization for Nuclear Research (Cern) has spent some 40m Swiss francs (£24m) on repairing the machine following the embarrassing mishap.
What was the accident that shut down the LHC?
The Large Hadron Collider started up on 10 September 2008. Engineers successfully sent proton beams in opposite directions around the 27km-long circular tunnel underground.
But just nine days later, a major accident in the tunnel forced Cern to switch off the machine.
An investigation confirmed that the cause of the accident was an electrical fault in a "soldering joint" linking two of the 1,200 "superconducting" magnets that steer sub-atomic particles around the LHC.
Superconductivity is the property, exhibited by some materials at very low temperatures, of channelling electrical current with zero resistance and very little power loss.
The fault triggered the release of helium within one of the magnets. This caused a pressure increase which lifted several of the superconducting magnets off their supports, breaking the interconnections between them.
This chain of events also triggered numerous magnet "quenches". A quench occurs when part of a magnet heats up, causing its superconducting properties to be lost.
The accident also caused about one tonne of helium to leak out into the tunnel. Liquid helium is used to cool the LHC to its operating temperature of 1.9 kelvin (-271C; -456F).
What repairs had to be made?
Engineers needed to replace 53 superconducting magnets in all. Some 54 electrical interconnections were fully repaired, while partial repairs were made to another 150 interconnections.
Over 4km of beam tube - the piping which carries sub-atomic particles through the magnets - had to be cleaned following the incident.
Cern also said it would fit a restraining system to some magnets and install hundreds of helium pressure release ports around the machine.
Could the same kind of incident happen again?
Cern officials say they are confident that the re-fit will prevent accidents of the kind that happened in September 2008.
To this end, they have also been upgrading the system which is designed to provide warning of magnet quenches.
Engineers have had to install hundreds of new detectors around the machine.
Among other things, the upgraded quench protection system is expected to improve monitoring of the interconnections between magnets.
Gianluigi Arduini, deputy head of hardware commissioning for the LHC, told BBC News: "It will allow us to constantly monitor the status of the interconnections. If there is any deterioration detected by the system, the powering of the magnets will be automatically stopped, preventing any damage."
Is the LHC in full working order now?
Almost. Since the incident in 2008, engineers have discovered the collider has hundreds to thousands of flawed electrical splices between magnets.
These do not need urgent repair, but they do limit the amount of current which can be safely put into the machine.
Officials have decided to put off repairing all the faulty splices. Instead, when the collider begins circulating proton beams, engineers have been told not to exceed a maximum "safe" limit of five trillion electron volts.
Secondly, tests have uncovered concerns about a number of the magnets themselves. All LHC magnets undergo a "training" process, in which engineers crank up the electrical current until the magnets quench.
After a few iterations, a stable configuration is reached where the magnet can reliably operate at the current it was designed for.
The magnets had been trained to their operating current before being lowered into the LHC tunnel. But once connected underground, some of them were found to have "lost" their training.
When the machine goes into its planned downtime in November 2010, engineers will attempt to re-train some of the magnets and, potentially, replace some proportion of the splices to ready the LHC for a push towards seven trillion electron volts - the LHC's maximum energy.