Geneva - On Sunday, more than 30,000 people are expected to visit what's described as the world's biggest scientific experiment when the European Centre for Nuclear Research (CERN) opens its underground doors for just one day before trying to unravel the secrets of the universe.

The Large Hadron Collider (LHC), the most powerful particle accelerator ever constructed, is located in a 27-kilometre circular tunnel 100 metres below ground level just outside Geneva on the Franco-Swiss border.

It has taken almost 15 years and cost 8 billion dollars to construct. CERN, already famous as the birthplace of the internet, is now poised to try to unravel the secrets of the universe and recreate what happened a trillionth of a second after the Big Bang, the point at which scientists say the universe was created.

"We are extremely excited about our experiment. It is the biggest experiment in the world and we hope we are going to understand all sorts of things like the origin of mass and what is the dark matter in the universe," theoretical physicist Professor Malcolm Fairbairn at CERN said.

The project will recreate what happens in nature all the time by sending protons crashing into each other. The main difference is that this time there are powerful detectors set up to monitor exactly what happens.

"It will be highest energy that man has ever created, but the key word is man, because in nature protons are smashing against each other all the time at much higher energies than those of the LHC," Fairbairn explained.

The LHC is inside a tunnel forming a 27-kilometre circle and is the world's largest piece of laboratory equipment.

Inside, protons, the smaller particles found in an atom, will be sent smashing into each other travelling at the speed of light, reaching an unprecedented energy level.

Scientists hope this will answer many questions, but will raise others in areas of physics that have so far been inaccessible.

The experiment has provoked opposition. Two scientists from Hawaii have lodged a challenge at a Honolulu court, claiming the accelerator could create a black hole that could destroy the Earth and even the universe.

Walter Wagner, who runs a botanical garden on Hawaii's Big Island, and Luis Sancho, a Spaniard, have asked for an injunction to prevent CERN from starting up the LHC until further safety assessments have been carried out.

Fairborn said the experiment only replicated what was happening in nature all the time.

"If there was any weird stuff happening, it would already be happening all the time when cosmic rays hit the Earth. So that's why we are not scared and we can quantify all these experiments mathematically."

CERN has done many calculations to ensure safety.

"We are very far on the safe side," Fairborn added. "Every time there is a new collier there is opposition."

The LHC is now complete and undergoing the slow process of cooling it down to minus 271 degrees Celsius, one degree colder than outer space, to allow for the accelerator's magnets to operate in a super- conducting state. It is expected to be fully operational by summer.

Sunday is the only chance for the public to get a glimpse of LHC before it is activated.

The number of visitors is strictly limited to 15,000, but organizers said there are many activities on the surface at access points to the ring to keep visitors occupied.

Press officer James Gillis said: "The tunnel is longer than London's Underground Circle Line. The particle detectors fill the caverns; one is the size of the nave at Westminster Abbey. Even if you see them everyday you still go, 'Wow.'" (dpa)