By Dr David Whitehouse
BBC News Online science editor
Scientists are unveiling one of the world's leading laboratories looking for "dark matter", in a cavern 1,100 metres underground.
In search of dark matter: Inside Boulby mine
Situated at the Boulby potash mine in the north of England, the 10-year-old facility has been upgraded and refurbished to lead the search for a vital component of the cosmos.
State-of-the-art detectors are being installed in the sub-surface observatory in the hope that its isolation and quietness will aid the search.
Dark matter is a fundamental though mysterious component of the Universe.
It could be in the form of sub-atomic particles that interact with normal matter only very weakly and are almost impossible to detect in a laboratory on the Earth's surface.
Boulby is one of the strangest and most remarkable laboratories on Earth - or rather below it.
To visit it, I had to don overalls, safety equipment and a helmet with a lamp. I had to sit through a briefing telling me the do's and don'ts, and how to handle an emergency.
Into a different world: At the liftshaft base
Then, wearing unfamiliar heavy safety boots, I walked from the locker room along a corridor to a huge metal door that hisses air as it is opened briefly. This is the start of the journey to the underworld.
Boulby is one of the world's deepest working mines and, situated near the coast, its tunnels reach far out under the sea.
For decades scientists have sought such mines, caves and caverns to put their detectors to try to trap sub-atomic particles from space.
Perhaps the most famous one was the tank of cleaning fluid in a South Dakota mine about 1,500 metres deep.
The tank detected neutrino particles from the core of the Sun. The observatory needed to be underground to shield it from confusing background radiation.
Scientists had puzzled over why our star seemed to produce fewer neutrinos than expected. Many considered such underground experiments to be flawed. But they were right; the problem lay with the neutrinos themselves and not the Sun.
The astonishing theory that these particles change their type en route to the Earth was confirmed in a Canadian detector in 2001.
Looking for wimps
Dark matter is believed to comprise 90% of the Milky Way and perhaps up to 99% of the Universe as a whole. Some of this matter could be in the form of cool stars, planets and black holes formed from collapsed stars.
Science underground: Inside the cavern
But the theory of the Big Bang puts a limit on how much of the Universe's missing content can be ordinary matter.
The leading dark matter candidates are heavy slow-moving particles known as Wimps (Weakly Interacting Massive Particles) that have been drifting through space since the Universe began.
If the Boulby scientists are lucky, they may see one pass by.
The UK Dark Matter Collaboration is a consortium of astrophysicists and particle physicists conducting experiments with the ultimate goal of detecting the rare events which would occur if the galactic dark matter consists largely of a new Wimp, called a neutralino.
Deep and dark
The descent into the Boulby mine is thrilling. This is no ride like in a lift down from the top of a tall building. The dive is smooth and rapid. The miners and engineers who accompany you seem almost bored by it.
From the base of the liftshaft, it is a short walk to the entrance to the laboratory.
Lights and cables are strung along the side of the tunnel but occasionally one passes a side tunnel that is completely dark; not the dark of a dark night, but a deeper dark - somehow indescribable.
I pulled a small flake of rock crystal protruding from the wall and brushed the dust off it. It was salty to the taste and scientifically that is good.
The natural salt is low in radioactivity so it will not confuse the detectors.
A shield of rock
The world a thousand metres below ground is an eerie one. It is an unchanging place, in one way isolated from the rest of the Universe, in another more a part of it than almost anywhere else on Earth.
Too much background radiation at the surface
Down here, the radiation and particles that bathe the surface of the planet are reduced to almost nothingness by the vast roof of rock above.
But this shield is but nothing to any particles of dark matter that might wander through.
The site and related facilities have now been refurbished and upgraded with a £3.1m grant that includes the construction of a new building on the surface and new lab facilities underground, creating what is almost certainly the best site in the world for dark matter research.
Boulby now hosts one of the world's most sensitive dark matter detectors called Zeplin.
A further experiment already making use of the new underground facilities is called Drift, which will be able to detect not only particle events but also the direction from which the particles come.
The first results from Boulby are tantalising, but the secret of dark matter will not be won easily. Getting the sensitive equipment built down here and working is just the start.