Boulby Underground Laboratory is an old potash mine in North Yorkshire.
Boulby Underground Laboratory, UK
At over 1000m deep, it's the deepest mine in Europe.
But these men aren't miners.
What they're searching for can't been seen, and has never been detected, but they're convinced it makes up about 85% of the mass of the Universe.
They're searching for dark matter.
The stars within galaxies, and the galaxies themselves, are moving much faster than we would expect from the gravitational pull of everything we can see.
The laws of physics suggest there must be more mass.
It's just not visible.
Scientists came up with the idea of dark matter.
Invisible particles that supply the missing mass.
These particles are thought to pervade our Milky Way, and every other galaxy in the Universe.
They interact very rarely with other matter, making them hard to identify.
They could pass through your body and you would never know.
This makes detecting them extremely challenging.
Scientists are working down mines, not only in the United Kingdom, but in America, Spain, Italy and France, because it increases their chances of finding dark matter.
Deep underground there's less interference from cosmic rays.
This radiation could give false signals when they try to detect the particles.
The protection of the mines means their equipment is more likely to find what they are looking for.
Nigel Smith, Sheffield University – "This is one of our latest detectors; this is a liquid xenon detector, surrounded as you see by a whole pile of lead to shield it from the radioactivity in the walls around us. What happens is that one of the particles...one of the particles that we are looking for hits this xenon, it gives off a flash of light and this process is known as scintillation and it's this flash of light that we're looking for."
This mine has not yet produced that flash.
The search for dark matter continues, so that scientists can explain, once and for all, what holds galaxies together.
Prof Tim Sumner, Imperial College London – "It is such a fundamental problem now, within both physics and cosmology, that finding the answer to that problem is... is going to be an enormous discovery."