The construction of a giant underground laboratory on the French-Swiss border that will help take physics into a new era has reached a major milestone.
A 2,000-tonne piece of machinery has been successfully lowered by crane into a man-made cavern 100m below ground.
The machinery is part of the Compact Muon Solenoid (CMS), one of four big experiments belonging to the world's most powerful particle accelerator.
A spokesman said: "Champagne corks are popping - it has all gone very well."
Called the Large Hadron Collider (LHC), the accelerator is designed to probe the limits of physics.
It is a powerful and complicated machine, which will smash particles together at super-fast speeds in a bid to unlock the secrets of the Universe.
The LHC comprises over 1,000 powerful magnets occupying a subterranean tunnel that runs in a ring for 27km.
The magnets carry two beams of particles around the ring at speeds close to the speed of light.
At certain points along this ring, the beams cross over, causing some of the particles to collide head-on.
Each of the four huge LHC experiments, including the CMS, sits near one of these crossing points.
These experiments, or detectors, will capture and measure new particles produced in the collisions.
These could point to new phenomena beyond the so-called standard model of physics - a framework to explain the interactions of sub-atomic particles.
The 2,000-tonne piece that was winched down the 100m shaft at Cern on Wednesday is the largest and most impressive segment of the CMS.
It took about 12 hours to lower the part into place.
It is called the Yoke Barrel 0, or YB0 for short, and forms the central "barrel wheel" segment of the CMS experiment. It is flanked on either side by two smaller wheels.
The YB0 element will house most of the critical inner parts of the experiment and is about 16m tall, 17m wide and 13m long.
"It is the largest of 15 pieces due to go down," said Jim Virdee, chief spokesman for the CMS science team.
"We were drawing all these things 15 years ago on a piece of paper. So to see them being assembled is exciting and partly emotional."
So far, eight of the 15 key elements that make up the CMS have been successfully lowered. YB0 was the ninth.
The experiment will now have to be assembled into its final configuration inside the cavern.
In its final form, the Compact Muon Solenoid will be cylindrical, 21m long and 16m in diameter and weigh approximately 12,500 tonnes.
The CMS is one of two general purpose experiments at the LHC.
It will aim to identify the elusive Higgs boson (known as the "God particle" because of its importance to the standard model of physics), look for so-called supersymmetric particles and seek out the existence of extra dimensions.
Science operations should begin early in 2008.