People in glass houses don't discuss sex: They had parties, TV and toilets, but no way out. Sally Silverstone tells Phil Reeves about her two years in a giant greenhouse

Most employers would put Sally Silverstone's curriculum vitae in the 'oddball' file. It begins in a fairly orthodox fashion: school in London; university at Sheffield; voluntary work overseas. But then you read that she has spent two years living in a gigantic greenhouse in the desert, isolated from the outside world.

In person, Ms Silverstone does not seem strange. When she talks about her stint in the Biosphere 2, a giant steel and glass structure in Arizona in which scientists are trying to create a closed ecological system, she does so with all the breathlessness of a Girl Guide showing off her first badge.

Ms Silverstone, 38, from Ilford in Essex, is one of two Britons in a group of eight researchers who in September clambered out through an airlock and returned to the outside world.

Their return, accompanied by a media fanfare orchestrated by the company running the dollars 150m ( pounds 100m) project, provoked some criticism. Several times oxygen had to be pumped in because levels fell too low; there were allegations that the structure leaked; there were rumblings that the enterprise, which attracts hordes of tourists, had more to do with flogging T-shirts and health food than science. But, as Ms Silverstone takes you through her daily routine, her flat Essex vowels intact despite more than eight years in the United States, you realise than none of this has dented her excitement about the cause.

'You can imagine what it was like to have walked past that door for two years, only ever peering out through the glass,' she says. 'But coming out was stunning. We just stepped out into Arizona and suddenly it was all over. There was this incredible rush of energy. The light looked totally different. The colours were very bright and the atmosphere smelt wonderful, although it was thinner. It was very different from the tropical air inside the Biosphere.'

With her denim shirt and jeans, tousled short hair and sensible sandals, Ms Silverstone resembles an unpretentious, maddeningly practical, field worker. Her manner is open, friendly, humorous. You can imagine her living in a mud hut, although this is far from the reality: the biospherians had smart, if small, two-storey apartments, complete with computers, televisions, flushing lavatories and showers, which used recycled water. They were in constant contact with the outside world, mostly by telephone. While the rest of the crew spent evenings reading, Jane Poynter, the other British biospherian, would have music jamming sessions with outsiders via a video link-up.

'I am an early riser,' Ms Silverstone says. 'So I would get up just before dawn, and spend a while thinking about what had to be done during the day. Then I would go off to milk the goats, and after that I would pick the day's fresh fruit and vegetables to take to the kitchen.

'I used to take the stuff fresh out of the garden. There would be squash, lettuce, tomatoes, peppers, papayas and bananas. I would take them to the cook. At 8am we had a meeting where we would go over what we were doing for the day. Often we were scattered around the Biosphere all day, so the meeting was important.

'We had a very small amount of animal produce. Meat was a big treat. We might have a small piece of meat once a week, but not for breakfast. That would be porridge (made from grain and sweetened with bananas) and usually potatoes and beans. We also had all these different herb teas. We used to make tea from mint and lemon grass and orange leaf.

'Then we would go off to do our chores. Everyone would work for a couple of hours on the agriculture; after that people would go off and see to their own areas. It was a very physical lifestyle, very physical.

'One of our most amazing crops was the bananas. I was worried about them because of our low light levels, but they did extremely well. So did sweet potatoes. Our rice varied depending on whether it was summer or winter, but it wasn't bad. It was all very experimental; we didn't know what would work.'

'For instance, we had terrible trouble with white potato. Everyone loves them, but we had a rampant species of mite which destroyed them. We could not use pesticide because we will not use any chemicals in the Biosphere except ones we know are non-damaging to the system, such as soap or small amounts of sulphur powder.'

At dinner the diet of vegetables was supplemented by a dessert of fruit or goat's milk. Fine for shedding a few pounds, but after two years it must have been the culinary equivalent of watching paint dry. The biospherians sometimes felt the same way. 'We used to have food fantasy sessions,' Ms Silverstone recalls. 'We would sit around after a meal and imagine what we would really have liked to have eaten for dessert.'

They did try to develop interesting recipes. For dollars 15.95 ( pounds 10), visitors to the Biosphere's tourist shops can buy a collection of Ms Silverstone's recipes, including one for biospherian doughnuts (potato, milk, bananas, yeast, wheat flour), which the crew ate without sugar or jam.

The biospherians' calorie intake became a critical issue. 'When we went in, we had no idea what our working levels would be like and how many calories we would need to support it,' says Ms Silverstone, the food systems manager. 'We took a well-educated guess.'

As members of the 'crew' (the biospherians and the project managers love Nasa-speak) began losing weight, it became clear that her estimate of 1,800 calories a day each was not enough. The team decided to increase their calorie intake to 2,200, although this meant supplementing their diet with supplies bought before their incarceration began.

There were, however, feasts to look forward to. These were elaborate affairs with which the crew celebrated the cycle of the seasons, birthdays or public holidays by quaffing mildly alcoholic home-made banana wine, and guzzling as much food as they could spare - including the kids born to their goats, and their two pigs, which the crew was forced to slaughter after realising that they ate too much.

So did four fit, healthy, single men and four fit, healthy, single women really sit in their home-made paradise, quaffing banana wine, without acting on that most natural of instincts? Did they cohabit for two years without - at least some - passionate relationships developing?

Ms Silverstone's features cloud over a little at this. The biospherians have been offered considerable sums of money by tabloid newspapers in the US to confess to steamy frolickings in the giant greenhouse. 'We always, always, always get asked that,' she sighs. 'We learnt to deal with it very early on. We just told people where to get off. That kind of publicity is of no interest to the public whatsoever.'

But shortly before the crew came out of the Biosphere, one of them, 46-year-old Mark Nelson, did drop a hint. 'People are people,' he said. 'Everything you might expect to happen with people has happened in here.'

Including, it seems, blistering rows. Ms Silverstone insists that the group got along well: 'We knew how not to push each other's buttons. If our time and energy was going to be taken up in endless petty squabbling, it would have been really hard to keep the project going.

'But there were lots of fights. They were usually about how to operate the place, and about what to do next. There wasn't real violence behind them, but there was a certain passion.'

Ms Silverstone's route to her life under glass was circuitous, and took her round the world. The daughter of a chartered surveyor, she left school in east London with three A-levels and went to Kenya to work as a volunteer in a children's home. She came back and took a degree in applied social science at Sheffield University, where she lived with friends in a farmhouse, rearing chickens and growing their own food.

After a stint at an institute for the mentally handicapped in Calcutta, she joined a voluntary organisation which dispatched her to a remote Indian village to help improve crop production.

There followed a move to Puerto Rico, to work on a project developing farming techniques in the rainforests without cutting down trees. By the time she heard of the Biosphere project, she was well versed in the business of producing food in rural communities. 'It was a 100 per cent tie-in with everything I had been interested in up until that point,' she says.

She has no immediate plans to re-enter the Biosphere, but is currently training the next crew - the project is scheduled to last 100 years. After two years trying to perfect self-sufficiency, after yards of column inches questioning the integrity of the project, Sally Silverstone barely pauses when you ask if she plans to continue her unorthodox life in the Arizona desert. 'Absolutely,' she says, grinning happily.

(Photographs omitted)