The hidden giants

Under land and sea, in vast subterranean laboratories, scientists are working to uncover the secrets of the universe, from 'dark matter' to global warming
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The Independent Online

The Kamioka Underground Observatory in Japan

The Kamioka Underground Observatory in Japan

A paradox in science is that the smallest and most ubiquitous things imaginable can often only be detected by some of the biggest and most expensive scientific instruments ever built. And most of them are so sensitive they need to be shielded in deep subterranean caverns. The Kamioka Underground Observatory in Japan is housed in an old mine located 1,000 metres below ground. At its core is a giant tank of ultra-pure water weighing some 3,000 tons and surrounded by 1,000 highly sensitive light detectors.

The water tank is 16m high and 15.6m in diameter. The 1,000 photomultiplier tubes fitted around the inside of the tank are there to detect the tiny flashes of pale blue light that may be emitted as a certain subatomic particle travelling at the speed of light collides with the nucleus of a water molecule. This particle - called a neutrino - has achieved almost mythical status in physics.

The Nobel laureate Wolfgang Pauli predicted the existence of the neutrino - which means "little, neutral one" - in 1931, but it took a further 25 years to prove its existence. Physicists calculated that neutrinos must be emitted in their trillions in space as a result of the nuclear fusion reactions of the Sun and stars, and the gigantic stellar explosions of supernovae.

The problem with neutrinos, however, is that they are so small and so electrically neutral that they pass straight through most things without ever interacting with them. Billions of these ghostly elements pass through each and every one of us every second without any effect.

Rock surrounding the Kamioka water tank shields the detectors from interfering cosmic rays, which allows the faintest interaction between a neutrino and a water molecule to be picked up. On 23 February 1987, the Japanese physicist and Nobel laureate Masatoshi Koshiba used the Kamioko instrument to detect a tiny fraction of the massive flux of neutrinos that passed through the Earth as a result of a distant supernova explosion. The instrument managed to detect just 12 neutrinos out of an estimated total of a thousand trillion that passed through the detector at that moment in time.

The Sudbury Neutrino Observatory in Canada

Neutrinos are important because they are the one subatomic particle that appears not to conform to one of the great universal laws of physics - the so-called Standard Model. At one time, neutrinos were thought not to have any mass at all; now, physicists believe they have some mass, but are much lighter than other subatomic particles. At another underground mine, near Sudbury in Ontario, a different neutrino detector has been designed to study one of the particle's most unusual properties - the ability to oscillate from one form to another.

The Sudbury Neutrino Observatory is located 2,000 metres below ground in another disused mine carved out of solid norite rock. This time, however, the 12m-wide tank at the core of the instrument is filled with 1,000 tons of heavy water, valued at £125m and on loan from the Canadian nuclear industry.

Heavy water has the same chemical composition as normal water, except that each molecule has two atoms of deuterium, an isotope of hydrogen with an extra neutron in its nucleus. The heavy water of the Sudbury experiment has helped scientists to show that neutrinos can, in fact, exist in more than one form, or "flavour".

Because heavy water can detect more than one flavour of neutrino, the Sudbury detector demonstrates that neutrinos from the Sun are emitted in one form and are transformed into another as they travel to Earth. This oscillation explains why other neutrino detectors have been unable to detect as many solar neutrinos as they should. The Sudbury machine, therefore, has helped to solve a puzzle known as the "solar neutrino problem".

But as one problem was solved, another was created. Under the Standard Model, neutrinos should not oscillate from one flavour to another. Further underground research is needed.

The UK Dark Matter Project in Boulby, Yorkshire

More than 1,000 metres below the rolling moors of north Yorkshire, set in the deepest mine in Britain, lies an experiment to find the missing mass of the Universe. The planets, stars, comets, asteroids and other visible objects of space only constitute about 10 per cent of the mass of the universe, as calculated using the laws of gravity. The rest is so-called "dark matter" and no one knows quite what it is.

One theory is that this missing mass consists of dead stars or other large objects that do not emit enough light or heat to be seen or detected by telescopes or other astronomical instruments. Another, popular, theory is that it consists of some kind of mysterious subatomic particle that has yet to be discovered. Physicists call these particles "wimps", short for "weakly interacting massive particles".

Last year, the UK Dark Matter Project opened a £3.1m laboratory in a working salt mine at Boulby in the hope of being first in the global race to detect the wimps that could account for the missing mass of the Universe. Like the neutrino detectors, the laboratory relies on interactions between subatomic particles and sensitive detectors buried so deep in solid rock that they are shielded from background radiation.

The theory is that as a wimp hits an atom of the gas xenon in one of the detectors, it sends the atom in a different direction, much like one billiard ball bumping into another. This sends out a tiny flash of light that can be recorded by the experiment's ultra-sensitive instruments. Several similar experiments around the world are looking for dark matter and the first person to find it will almost certainly win a Nobel prize.

The Large Hadron Collider on the Franco-Swiss border

In terms of its sheer volume, the Atlas experiment at the Large Hadron Collider (LHC) is the largest underground instrument ever built to detect subatomic particles. When fully constructed it will be five storeys high and weigh some 7,000 tons. It is being built inside a giant subterranean cavern big enough to house Canterbury Cathedral. Atlas is one of five experiments that make up the LHC, an atom-smasher operated by Cern, the European organisation for nuclear research that spans the Franco-Swiss border near Geneva.

Unlike the search for neutrinos and dark matter, which rely on passive experiments, the LHC is a stupendously ambitious and complex smasher of atoms that actively creates weird particles and events. At its heart is a circular, underground tunnel some 27km (16.8 miles) in circumference, where two beams of protons travelling in opposite directions are accelerated to intense energy levels before being deliberately slammed into one another.

Huge, helium-filled magnets cooled to minus 273C will help to control the movement of the two proton beams to within a thousandth of a millimetre. The magnetic force generated will be 500 tons per metre, equivalent to the total take-off thrust of a jumbo jet. The aim is to collide not just the two proton beams, but the much smaller subatomic constituents, known as quarks, in a proton. Scientists hope that by doing this it should be possible to shake out the elusive Higgs boson - otherwise known as the "God particle" because of its importance in understanding the fundamental nature of matter, creation and the Universe.

Cern's LHC is effectively re-creating the conditions that existed when all matter came into existence during the Big Bang. When Atlas is fully operational in about three years, it will monitor some 800 collisions a second and the Higgs boson, if it exists, should make an appearance at least once a day. If the Higgs particle does not exist, however, physicists will have to devise a new theory to explain one of the fundamental laws of nature.

Arctic Coring Expedition, near the North Pole

This summer, at a point just 200 miles from the North Pole, marine scientists began one of the most ambitious underwater projects ever carried out on the seabed. Using three icebreakers working in unison, an international team has been attempting to drill through the moving ice floes of the Arctic Ocean and into several hundred metres of the seafloor below. They hope to collect a core of sediments that will provide hidden details of the past 50 million years of the Earth's climate.

The Integrated Ocean Drilling Program - a huge international project to investigate the history of the Earth - has never before attempted to drill boreholes in water less than 200m deep, nor has it tried to do so in polar oceans covered in ice sheets. This August, the Swedish icebreaker Vidar Viking tried to do just that when it began to drill below the Arctic Ocean for the first time, with the help of two other icebreakers, the Danish Oden and the Russian Sovetskiy Soyuz, which were there to protect the drillship from being crushed by moving pack ice.

Scientists from the British Geological Survey have been taking part in the drilling operation, which was controlled by satellite navigation to keep the drillship dead on target over its borehole. Helicopters warned the ships' captains of any pressure ridges that were forming in the surrounding ice that could snap the drill in half.

The plan was to collect sediment cores from the Lomonosov Ridge, a piece of uplifted crust crossing the Arctic Ocean underneath the North Pole and stretching from Siberia to Greenland. Such sediments should hold a record of the Earth's climate stretching back to the time when the Arctic was much warmer than it is today. Scientists hope to explain why the Earth goes through regular ice ages followed by warm, interglacial periods. It will put climate change into a wider, historical context.

San Andreas Fault Observatory at Depth, California

Drilling a deep hole through solid rock is never easy, especially when it's done at a 50 degree angle through a geological fault line separating two huge tectonic plates. Yet this is precisely what the US Geological Survey is aiming to do with its plan to monitor the infamous San Andreas fault for the telltale signs of the Big One, an earthquake so powerful that it could plunge much of California into the ocean.

When it is finished, the borehole will be three miles long. The first 1.5 miles of its length will be drilled vertically into the Pacific Plate. From there, the drill will be angled to run for another 1.5 miles until it breaks through the fault line and becomes embedded in the North American Plate, which is moving in the opposite direction to its Pacific neighbour.

The drill hole will be the first to penetrate two actively sliding plates and will enable scientists to build a seismic observatory more than two miles below ground, where temperatures soar to 135C. It will be the first time that scientists will have the equipment and instruments to conduct round-the-clock monitoring of seismic activity from inside an active earthquake zone.

Strain gauges, thermometers, pressure monitors and other instruments lining the borehole will provide a constant stream of information about what happens in the bowels of a fault zone. The idea is to be able to predict one of the most destructive forces in nature. As Stanford University geologist Mark Zoback explains: "By making continuous observations directly within the San Andreas fault zone at depths where earthquakes start, we will be able to test and extend current theories about phenomena that might precede an impending earthquake."

The last of the sensors will not be in place until 2007, long before, it is hoped, the next Big One.

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