Media playback is unsupported on your device Media caption Engineer Katy Foraz shows Pallab Ghosh how to upgrade the Large Hadron Collider

Engineers have begun a major upgrade of the Large Hadron Collider (LHC).

Their work should double the energy of what's already the most powerful particle accelerator in the world.

BBC News is the first to be allowed to see inside the LHC - on the French-Swiss border - to watch the work being carried out.

Scientists believe the upgrade will enable them to discover new particles which will lead to a more complete theory of how the Universe works.

A project leader with the LHC's Atlas experiment, Dr Pippa Wells, told BBC News that there was much more to come from the LHC.

"The past two years have been the most exciting in my time as a particle physicist. People are absolutely fired up. They've made one new discovery (the Higgs) and they want to make more discoveries with the new high energies that the upgrade will give us. We could find a new realm of particle physics."

I was taken by the technical coordinator for the upgrade project, Katy Foraz, and Cern's UK communications manager Stephanie Hills, to one of the many access points to the LHC's underground tunnels.

LHC Upgrade Replace 10,000 connections

Install 5,000 insulation systems

10170 leak tests

18,000 electrical tests

Total cost £70m

We entered a lift shaft with two buttons marked zero and minus 1. Katy hit minus 1 and we made our way 100m below the surface.

As we exited the lift, we walked to a large heavy green door that we strained to open.

As we went through, it was like entering Aladdin's cave.

No jewels or gold - but one of the largest and most complex machines ever built. A bright blue superconducting beamline stretches into the distance - around it are gleaming precision instruments to make the line one of the coldest places in the Universe.

In front of me, engineers were replacing some of the first connectors. In all, 10,000 will need to be changed. Eight hundred people are involved in this project, which will cost £70m.

The tasks also include testing and replacing some of the LHC's main dipole and quadrupole magnets, which are used to bend the paths of the particles and keep them tightly bunched; conducting tests to detect any irregularities in the magnets or imperfections in the electrical insulation; and a range of other work to improve the machine.

We are always at the limit of what we know in terms of the technology. It is very exciting for an engineer to be close to all these new technologies Katy Foraz, LHC Upgrade coordinator

The LHC is known for its cutting-edge science. But as we walked to this scientific wonderland, Katy told me that people often forget that the particle accelerator is also on the cutting edge of engineering. After all, creating the conditions of the beginning of the Universe is no easy feat.

"We are always at the limit of what we know in terms of the technology. It is very exciting: as the coordinator. I have access to all the technologies and they really are at their limit in terms of superconductivity cryogenics. It is very exciting for an engineer to be close to all these new technologies," she said.

Katy and her team of engineers are calling the work an "upgrade". But critics say it's a "repair".

As we walked passed a team replacing a damaged connector, Cern's Stephanie Hills was quick to respond to the charge that this expensive refit is putting right a mistake that should not have been made in the first place.

"Nobody has ever done this kind of technology before. Everything from the most basic welding to the most complicated beam diagnostics is pushing the boundaries of technology, and sometimes these things just don't go right simply because we don't know how it's going to work," she told me.

"You can see in front of us the way that we're managing the upgrade is meticulous. There is lots and lots of quality control, making sure that everything's absolutely spot-on so that when we turn the machine back on we are absolutely ready for some more fantastic scientific discoveries."

The damage was done shortly after the switch on of the Large Hadron Collider in September 2008.

Media playback is unsupported on your device Media caption The LHC upgrade will enable it to discover new particles. Pallab Ghosh explains how this will lead to a radical change in our understanding of how the Universe works.

Nine days later, it broke down because the connections between the superconducting magnets simply could not take the current running through them.

It took a year and £24m pounds of taxpayer's money to repair the damage. Even then it could only operate on half power. That was enough to discover the much sought-after Higgs Boson.

The LHC is more than just a one trick pony, We hope to find something completely new that will change our understanding of the Universe. Dr Pippa Wells, Atlas Project Leader

Those in charge made a pragmatic decision. They decided to press ahead, and to keep their funders happy.

To their joy and relief, scientists found their prize last summer. And so at the beginning of this year it was politically possible for Cern to begin the long shut down to fix faulty connections.

Back above ground, students from around the world are shown the Atlas control room, one of the places where data from the LHC will be gathered when it is switched back on. That is something for the students to look forward to - because after the upgrade, the beams will be crashing into each other at twice the energy.

This will enable researchers here to move on to their ultimate goal: to find evidence of "new physics", which they believe will lead to a new, more compete theory of sub-atomic physics.

The discovery of the Higgs last year was the end of a successful chapter of late 20th Century physics.

This was the development of the current theory in the 1960s and 70s called the "Standard Model".

This theory says that most of the forces of nature, the objects around us and our own existence, are all down to the interaction of the Higgs with 16 other particles. It successfully explains how electricity, magnetism and light operate.

Since then, all the particles predicted by the Standard Model have been discovered - including most recently the Higgs.

The problem though is scientists known this theory is limited. It explains extremely well the world around us, but it cannot explain the way most of the Universe behaves.

Physicists hope that by operating at full power, the LHC will be able to find evidence of so-called supersymmetric particles. These are like the particles in the Standard Model - but more massive.

One form of supersymmetry predicts that there should be five Higgs bosons, which are each slightly different. The first order of business for LHC scientists when the collisions resume in 2015 will be to test the Higgs that's been discovered, to see if it shows any of the properties predicted by supersymmetry, according to Dr Wells.

"The LHC is more than just a one trick pony," said Dr Wells. "It wasn't designed to find just the Higgs. We hope to find something completely new that will change our understanding of the Universe. We are on the threshold of finding many more new particles."

Follow Pallab on Twitter @bbcpallab