The Welsh scientist in charge of the world's biggest experiment says it will not result in another "Big Bang".
Dr Lyn Evans, 63, from the Cynon Valley town of Aberdare, set off the Large Hadron Collider (LHC) border with a click of his computer mouse.
There have been some warnings the experiment on Wednesday could wreak disaster on the entire world.
But the physicist and project leader said beforehand: "Don't worry, there will not be a tsunami in Cardiff Bay."
Dr Evans confidently dismissed the claims by some scientists that the LHC switch-on in front of millions could herald the end of the world, or any other disaster.
He said it was a pity that they had "stirred up" such controversy. "It's total hogwash. We're not in the business of science fiction, we are in science."
The LHC, which Dr Evans called "a discovery machine, the most sophisticated scientific instrument of our time," will smash two beams of particles head-on at super-fast speeds, recreating the conditions in the Universe moments after the Big Bang.
Scientists hope to see new particles in the debris of these collisions, revealing fundamental new insights into the nature of the cosmos.
The LHC's purpose is "fundamental research"
Dr Evans is now just over a year away from retirement after almost four decades devoted to the research.
He said while it was hoped it would give clues to the origins of the universe, they did not know exactly what results the £4.4bn experiment would provide.
It is taking place in a purpose-built tunnel under the French-Swiss border and involves hundreds of scientists.
He said the research was "very fundamental" but could also lead to some exciting offshoots.
Dr Evans grew up in a coalmining family in Cwmbach, and attended Aberdare Grammar School, where his early interest in science blossomed and he went on to take a degree and PhD in physics at Swansea University.
In 1969 he made a three-month visit to the European Organisation for Nuclear Research (Cern) project in Switzerland.
He and his wife Linda, who is from Trecynon, also in the Cynon Valley, and their family have stayed there ever since.
I believe his success at Cern will be an inspiration to young Welsh scientists and engineers
First Minister Rhodri Morgan on Dr Evans
But there are still strong links with his Welsh roots.
Dr Evans is a honorary fellow of Swansea University where he is looking forward to return in December to deliver a lecture.
He is also a professor of physics at Imperial College in London and he said he imagines that role will still keep him busy when he retires from Cern in 18 months.
He is also a keen watcher, largely via satellite TV, of the fortunes of Wales' resurgent rugby team.
First Minister Rhodri Morgan paid tribute to Dr Evans, saying: "I'm proud that Wales is playing such a key part through Dr Evans.
"He has a brilliant scientific mind which I'm delighted to say has been nurtured throughout the Welsh educational system culminating in his graduation from Swansea University in 1966 which he followed with a PhD.
"I believe his success at Cern will be an inspiration to young Welsh scientists and engineers.
Dr Evans is passionate too about the importance of encouraging youngsters along the path his career has taken.
"All over Europe there are problems that there are not enough people going into science," he added.
He will have the eyes of the world's media focussed on the underground laboratory on Wednesday.
"I'm dreading it," Dr Evans admitted.
"Making a particle accelerator work is not trivial, making it work with the whole media of the world on your back is going to be tough."