David Shukman heads deep underground to take a look at the LHC's tunnel
Plans to begin smashing particles at the Large Hadron Collider (LHC) may be delayed after a magnet failure forced engineers to halt work.
The failure, known as a quench, caused some of the LHC's super-cooled magnets to heat up by as much as 100 degrees.
The fire brigade were called out after a tonne of liquid helium leaked into the tunnel at Cern, near Geneva.
The LHC beam will remain turned off over the weekend while engineers investigate the severity of the fault.
A spokesman for Cern told the BBC it was not yet clear how soon progress could resume at the £3.6bn ($6.6bn) particle accelerator.
While the failure was "not good news", he said glitches of this kind were not unexpected during testing.
The first beams were fired successfully around the accelerator's 27km (16.7 miles) underground ring over a week ago.
Superconducting magnets are cooled down using liquid helium
The crucial next step is to collide those beams head on. However, the fault appears to have ruled out any chance of these experiments taking place for the next week at least.
The quench occurred during final testing of the last of the LHC's electrical circuits to be commissioned.
At 1127 (0927 GMT) on Friday, the LHC's online logbook recorded a quench in sector 3-4 of the accelerator, which lies between the Alice and CMS detectors.
The entry stated that helium had been lost to the tunnel and that vacuum conditions had also been lost.
It added that the Cern fire brigade had been called to the scene.
The LHC has been in construction for some 13 years
The superconducting magnets in the LHC must be supercooled to 1.9 kelvin (-271C; -456F), to allow them to steer particle beams around the circuit.
As a result of the quench, the temperature of some magnets in the machine's final sector rose dramatically.
A spokesman for Cern confirmed that it would now be difficult, if not impossible, to stage the first trial collisions next week.
Further delays could follow once the damage has been fully assessed over the weekend.
The setback comes just a day after the LHC's beam was restored after engineers replaced a faulty transformer that had hindered progress for much of the past week.