Now, the Large Hadron Collider is the world's biggest atom smasher, but as it turns out, it's not big enough. The European Center for Particle Research, CERN, is unveiling details of a new particle accelerator today, something three times larger and twice as deep. Our science correspondent, Palab Gosh, has been to Switzerland to find out why they need an upgrade.

Wow. Welcome to the Large Hadron Collider.

It's huge. It's massive. I can tell you, after 10 years working here, coming through this tunnel nearly every day, I still get that wow as I look at all of it.

This experiment has been going on for more than a decade. It's made some important discoveries, such as a particle called the Higgs boson.

This is one of the detectors that discovered the Higgs 12 years ago. It was an incredible scientific achievement. But the LHC was built to do much more than that. It was supposed to discover brand new particles that would change the theory of physics. It hasn't. So in that sense, it's failed.

And that's why they need a larger machine. The plan is to build what's called the FCC, the Future Circular Collider, next to the existing accelerator. It will be at least twice as deep and measure nearly 91 kilometers. That's around 56 miles. Inside, particles will travel much further than they currently do. They're pulsed by an electric field and stronger magnets, which make the particles collide with much greater force, and hopefully, revealing far more.

There are so many outstanding questions in fundamental physics today and in our knowledge of the universe, its structure, and its evolution for which we have no answer. And so we need more powerful instruments to be able to addressing those questions.

Thousands of scientists here are hunting for the tiny particles that are contained in the atoms that make up the world around us. Professor Mitesh Patel has spent his entire professional life searching for them.

I think for me, this is really about exploration, to be able to look for something genuinely new. If you're going to go and explore the unknown, then of course, you don't know what you're going to find, and you can't guarantee a particular outcome. But that doesn't It doesn't mean that it's not important for us to look.

This enormous underground lab lies hidden away beneath the Swiss French Alps. The plan is to stretch it even further. So detailed environmental assessments are being carried out on the impact to the community.

We care a lot about the environment. It's very important, especially excavating rocks or water. It can be some concerns. So we have a lot of questions to soon, and we expect, of course, to address all these questions.

The first stage of the new Collider won't be fully operational until 2045. If it gets a go ahead, there'll be huge environmental, technological, and scientific challenges to overcome. Then, of course, there's the cost, up to £17 billion.

You're going to so much effort digging all these tunnels, spending so much money to smash particles together. And so what's the point?

Scientific exploration. I think it's almost like asking, what's the point of art or music? I think humans have this curiosity of finding what's out there and what is... How does the world around you work? And FCC will help us answer some of those questions.

The researchers here feel that they're within reach of finding new particles that really make the universe tick. One more heave, they say, and they could make one of the biggest breakthroughs of all time. Palab Gosh, BBC News, CERN.