Depth of knowledge

A disused gold mine in the hills of the US mid-west, long ago stripped of its last remaining riches, may soon yield scientific discoveries far more valuable than the glittering precious metals that it once contained in abundance.

If a group of US scientists are given the go-ahead, the Homestake gold mine in Lead, South Dakota, will be converted into the world’s biggest, deepest and most sophisticated underground laboratory. It could, claim its proponents, be used to probe some of the most compelling mysteries of the universe.

Headed by Prof Kevin Lesko, an astrophysicist at the Lawrence Berkeley National laboratory in California, the team of physicists and engineers behind the proposal is to receive $15m (£7.4m) from the National Science Foundation (NSF) to develop a detailed technical design for its Deep Underground Science and Engineering Laboratory (DUSEL).

Lesko hopes that he will then be granted permission to begin opening up the mine for experiments.

It seems somewhat counter-intuitive that by burrowing beneath the ground we can discover more about our place in the cosmos. Yet the deep, dark places of the planet, shielded by the earth’s crust from the noisy clatter of cosmic rays, provide the perfect conditions for detecting the pin-drop signatures of sub-atomic particles that could shed light on physics’ most enduring questions.

There are other underground laboratories around the world — most notably the Gran Sasso facility in Italy — but Homestake’s DUSEL would, said Lesko, effectively double the world’s available amount of underground research space.

Homestake, founded during the black hills gold rush of the 1860s by mining magnate George Hearst, was until it closed in 2002 the largest and deepest mine in the western hemisphere. Its labyrinthine 500 mile-long (804km) network of tunnels reach more than 8,000ft (2,438m) below the surface and at its darkest depths the walls are 57°C (135°F) — a reminder that the earth’s core is just that little bit closer.

As well as investigating some of physics’ most beguiling conundrums such as the nature of dark matter and the properties of the ghostly neutrino particle, Lesko said the lab will also host experiments from a range of other scientific fields.

With direct access to geological structures and tectonic processes, for example, geologists are queuing up to use the facility to gain a unique insight into the behaviour of sub-surface rock, possibly leading to new earthquake prediction technologies.

Biologists, too, keen to study the strange micro-organisms that inhabit the rocks, hope to gain a greater insight into the origins of life on this, and possibly other, planets. It is the proposed multi-disciplinary nature of the facility that will, said Lesko, set it apart from other underground laboratories.

But before the DUSEL can begin to address these questions, Lesko’s team must turn its attention to the considerable technical challenge of turning a dark, dank, disused mine into a facility that is safe and comfortable enough to accommodate hundreds of scientists and, more importantly, to house some of the most sensitive scientific instruments imaginable.

According to Richard DiGennaro, project manager and system engineer for the project, though much of the core infrastructure — in the form of the mine’s numerous tunnels and shafts — is in place, plenty of work needs to be done to make the underground environment suitable for the type of physics experiments DUSEL will host.

The plan is to construct three campus levels: a near-surface laboratory at a depth of about 300ft, a mid-level lab at 4,850ft and a deep facility at 7,400ft, an undertaking that will, explained DiGennaro, involve excavating about 200,000 cu yds (152,910 cu m) of rock.

One of the most immediate problems will be dealing with the fact that a good portion of the mine is flooded, and DiGennaro anticipates it taking about 18 months to pump water out of the lower levels. Although a natural inflow of about 700 gallons of water/min makes the mine pretty dry compared with other disused pits, DiGennaro said the team is keen to get pumping as quickly as possible, before rising water levels make the cost of a huge pump-out prohibitively expensive.

Once the various shafts and tunnels have been dried out the engineering team will then have to turn its attention to installing the systems and utilities that will transform Homestake into a spanking new laboratory — a challenge that DiGennaro compares with the construction of a skyscraper.

While the ventilation and power distribution systems used in the facility will be borrowed from the mining industry, many of the other requirements will be far more extreme than the kind of systems typically deployed in a mine.

DiGennaro explained that the requirement for water purification systems, clean room facilities and climate control technologies has more in common with the semiconductor industry than mining. ‘There will be many specialised systems such as you see in surface labs — one of the challenges is developing on a very large scale,’ he said.

Only when all of this has been achieved can the experiments begin in earnest and one of the areas in which the facility is expected to make the greatest contribution is in the field of neutrino research.

Neutrinos are sub-atomic particles of almost no mass that stream forth from the thermonuclear reactions in the Sun or exploding stars. While they are one of the most abundant particles in the universe their almost negligible mass makes them difficult to detect. But physicists believe that a greater understanding of these ghostly particles could provide us with priceless information about the origins of the universe.

This will not be the first time that Homestake has been used for neutrino research. In 1965 Nobel prize winning physicist Raymond Davies used it to detect neutrinos from the sun for the first time. He constructed a tank that he filled with 378,000 litres of perchloroethylene. Neutrinos passing through the tank changed some of the chlorine atoms to argon.

But things have moved on since Davies’ day and, according to Lesko, the new facility will house the combined capability of all the world’s leading neutrino laboratories in one place: a sort of hypermarket for astrophysicists.

While experiments in recent years at some of the world’s other underground neutrino labs have confirmed that what was once thought to be a massless particle does, indeed, have a small amount of mass, experiments at Homestake are expected to take neutrino research to the next level.

By building a super-sensitive neutrino detector inside the lab, Lesko hopes researchers will be able to zero in on the exact mass of neutrinos by looking for a rare nuclear transformation called neutrinoless double beta decay. By figuring out the precise mass of neutrinos, it will be possible to predict when they will slow down and their epic journey through space will come to an end. This, said Lesko, has critical implications for the formation of superclusters of galaxies, the largest structures in the universe.

As well as neutrinos, Homestake’s astrophysicists will also be carrying out studies into dark matter, the mysterious form of matter that is invisible to current means of detection but which makes up 23 per cent of the mass and energy of the universe. If, as scientists suspect, it is an unknown particle, ultra-sensitive devices may be able to see the signal produced when a dark matter particle hits an atom in a detector.

In a somewhat more down-to-earth application, some reports have suggested that the military may be interested in using Homestake’s sensitive instruments to spot the tell-tale signs of covert underground nuclear weapons tests carried out by hostile nations.

But it is not just physicists that are excited by the prospect of the laboratory. With its easy access to many levels of the earth’s crust, geologists hope to use the lab to study the natural bending and straining of underground rocks, and gain a better understanding of how earthquakes form and what seismic signatures to look for before they occur.

An improved understanding of the dynamics of the underground world is also expected to yield new technologies for the prediction of rock conditions and behaviour. Properties of rock vary wildly and an improved understanding would remove some of the uncertainties of underground engineering and help reduce the high costs involved.

This could enable engineers to develop safer methods of designing and constructing all different types of underground structures — from transport systems to waste disposal facilities.

According to Lesko one of the most immediate tangible benefits of this area of research will be its contribution to the development of carbon sequestration technology, the idea of safely burying global warming gases such as carbon dioxide underground.

Biologists are also enthusiastic about the facility, and keen to study the strange and unique micro-organisms that have clung on to life in the inhospitable depths of the mine. According to Lesko, historically the only window scientists had on this world was provided through isolated bore-hole experiments.

The Homestake facility will give them an opportunity to contemplate the environment in its entirety.

‘This would give us a dedicated facility where we could focus in three dimensions down to what’s thought to be limits of life where the rock reaches 130°C,’ said Lesko.

‘Conditions deep in the earth’s crust are similar to those experienced by the first organisms to inhabit the planet — and these experiments could shed light on the origins of life here.’

Researchers believe that the underground world could also provide valuable clues on where to look for life on other planets.

But for now, the attempts to answer these questions lie in the future and Lesko’s team is spending the next three years working on its technical design. This will be reviewed, and ultimately congress will make a decision as to whether the $300m super-lab is given the go-ahead. Lesko is brimming with confidence and hopes it will be open for business as soon as 2010.

In the meantime South Dakota has given his team the green light to develop a $65m interim laboratory at the mine. Lesko hopes to begin conducting the first underground experiments in this 4,850ft-deep facility by next autumn.

If the larger laboratory does get the go-ahead, it is the multidisciplinary nature of the facility that its proponents are most excited about. Uniquely among its rival underground research spaces the DUSEL will play host to a dizzying mix of astrophysicists, evolutionary biologists, geoscientists and engineers, all pursuing their research interests with others from different backgrounds. And it is the unpredictable synergies of these different areas of research that could be most exciting.

Sidebar: More underground labs

The world’s largest underground laboratory is Gran Sasso in Italy. the facility, 120km from Rome, is located off a 10km-long freeway tunnel that crosses the Gran Sasso mountain range. Used exclusively for particle physics experiments, it consists of service tunnels and three large experimental halls, each about 100m x 20m x 18m.

While Gran Sasso is the largest, the deepest lab is Canada’s Sudbury Neutrino Observatory (SNOLAB) located 2km deep in Sudbury, Ontario.

A former nickel mine, originally developed to study neutrinos, the lab recently received $8m (£4m) to build a cryopit — a large cavern in which it will store low-temperature liquids and gases required for dark matter research.

However, many of the biggest breakthroughs in neutrino research over the past decade have been made by scientists working at the Kamioka underground laboratory in Japan, in an abandoned lead and zinc mine. Experiments at the 1,000m-deep lab confirmed that neutrinos, long thought to be weightless, do have a mass.

In the UK, the Dark Matter Collaboration is using the still-functioning Boulby Potash mine near Middlesborough to search for the weakly interacting massive particles (WIMPS) that some claim could form much of the Universe’s dark matter.