Gran Sasso is a mountain range, a national park, and a particle physics laboratory in central Italy. The mountains are the highest in Italy, south of the Alps. Located around 100km northeast of Rome, the 30km long chain is part of the Apennines, which form the backbone of Italy. The ash-coloured peaks are covered with snow for much of the year; and the lower slopes are a mix of ancient forests and rocky plains. The massif was declared a national park in 1995. The laboratory lies deep below the mountains, accessible by a ten-kilometre motorway tunnel, which cuts through the range.

L'Aquila

L'Aquila is the nearest city to the park, located at the foot of the Apennines at around 700m above sea level and easily reached from Rome. It's a modern city by Italian standards, founded in 1242, and full of sombre medieval stone buildings overlooked by mountains. It has a good selection of restaurants, supermarkets, and mountaineering equipment shops, among others. There's also an interesting museum inside a 16th Century castle, containing displays of local art and archaeology and the remains of a prehistoric mammoth.

Fonte Cerreto to Corno Grande

From L'Aquila you can take bus number six up the mountain. The bus goes as far as Fonte Cerreto, a resort complex at 1120m, with three hotels, a campsite, and a few bars. From there a cable car runs up to Campo Imperatore¹, a skiing and mountaineering resort at 2130m. There's a hotel there, where Mussolini was briefly imprisoned by his government before being rescued in a daring raid by German paratroopers in World War II.

To get higher you must go on foot. The highest summit is the Corno Grande at 2912m, a tough climb only possible in Summer. There are hostels for hikers and footpaths throughout the range, although these are sometimes no more than lines on a map, with a few marker posts.

Trekking this high up is not for the inexperienced. Some routes go across 45 degree slopes for many kilometres and it's easy to slip on the scree, snow, or ice. Even on the lower slopes it's important to be prepared, as the terrain is uneven; and the weather can change from bright sunshine to a heavy downpour very quickly.

Assergi

Walking downhill from the resort, you reach the entrance to the motorway tunnel and the laboratory offices. Further down lies the village of Assergi, an interesting settlement inhabited by around 50 sheep farmers and a similar number of particle physicists.

Families who have lived in Assergi for generations still have historic rights to graze sheep, hunt, and collect wood from the forest. The shepherds let their flocks graze on the mountain slopes during the summer, moving them south to other pastures for the winter, a practice they have followed for many centuries. Apart from this, there is little agriculture; and vast areas of the mountains are total wilderness. Although the higher altitudes are mostly wild, rocky outcrops, the lower slopes contain substantial woodland with much wildlife, including chamois, wild boar, and wolves.

At weekends, the road up the mountain is packed with skiers from Rome. Once you get away from the resorts, however, you can walk all day without meeting anyone... except, perhaps, the occasional shepherd or lost scientist.

Laboratori Nazionali del Gran Sasso

The Gran Sasso Laboratory was constructed in the 1980s, at the same time as the motorway tunnel. Halfway along the tunnel, a slip road leads to a series of large metal doors, which open to reveal an extensive laboratory complex in three huge halls bored right into the heart of the mountain. The local residents in Assergi will tell you stories about the secret military purpose of the laboratory. But what attracts hundreds of physicists from around the world, however, is the incredibly low background radioactivity found there.

The surface of the Earth is continuously bombarded with high energy particles from outer space called cosmic rays. While we have evolved to live quite happily with them, they are a real nuisance to scientists working on certain experiments known as 'rare event searches'. These involve trying to detect a particular event, say a beta particle emitted by a radioactive atom, which only happens very occasionally, say once a day. It's impossible to detect these events on the surface, as any particle detector registers around ten events per second just from cosmic rays. However, the 1.4km of rock above the Gran Sasso laboratory effectively absorbs all cosmic rays, leaving a near perfect quiet environment, where physicists can carry out precision measurements in peace.

There are around 30 different experiments taking place in the laboratory's three halls, researching many different areas of physics. These include:

• Neutrino astronomy - Neutrinos are tiny particles which hardly ever interact with atoms, so they usually pass straight through the bulk of Gran Sasso and carry on straight through the Earth. However, their interaction rate is not zero... just very small, so it is possible to detect them; and several experiments in the laboratory do this. Neutrinos are produced by the sun. A big mystery in particle-astrophysics is why these neutrino observatories have, so far, only detected a third of the number predicted. One theory is that neutrinos can change from one type of neutrino to another when moving over long distances. But the current experiments detect only one type of neutrino. There are experiments running to test this hypothesis. Recent results from the Sudbury Neutrino Observatory, located in a nickel mine near Sudbury, Ontario, Canada, provide strong evidence for this theory.

• Dark matter - One thing astronomers are pretty sure about, is that they can only see a small fraction of the total mass of the Universe. The remainder is made up of invisible dark matter. It is believed that at least a large percentage of this dark matter consists of exotic particles, which are very heavy. But, like neutrinos, they hardly ever hit an ordinary atom. Unlike neutrinos, however, these particles have not yet been directly detected. There are a few experiments in Gran Sasso trying to do this.

• Double beta decay - Beta decay is when a neutron in an atom changes to a proton and emits an electron. Double beta decay is when this happens to two neutrons in the same atom simultaneously. This is an extremely rare event. Studying exactly how it happens could tell us a lot about the fundamental processes governing how particles interact. So, many Gran Sasso scientists are interested in this.

• CERN neutrinos to Gran Sasso - This is a project currently being constructed. The idea is to generate a beam of neutrinos at CERN, the European Laboratory for Physics in Geneva, Switzerland², and send this beam straight through the Earth to Gran Sasso, where a small number of them will be detected. This experiment will allow scientists to test the changing neutrino hypothesis and study these particles in more detail.

It is possible to join a tour of the laboratory (in Italian). However, unless you have a great love of underground halls full of machinery, there's not a great deal to interest the average tourist.

Working in the laboratory can be a frustrating experience, because you're staying in a beautiful, remote region, with some of the best hiking trails and ski slopes in Italy... and you spend all day working inside a motorway tunnel.