In this moving film Horizon follows the Loughran family in their fight to save the life of their daughter Sheila who suffers from cystic fibrosis. They lost their youngest daughter Ann to the disease in 1974 at the age of 15, and now as the health of their third daughter Sheila deteriorates, they must face the prospect of losing a second child. The current shortage of donor organs means that Sheila's only hope of survival is a rare and controversial operation that requires her two surviving siblings to undergo an arduous and potentially fatal operation.

On a barren desert in South America lies one of the greatest archaeological puzzles in the world. Etched in the surface of the desert pampa sand are hundreds of straight lines, geometric shapes and pictures of animals and birds - and their patterns are only clearly visible from the air. They were built by a people called the Nasca - but why and how they created these wonders of the world has defied explanation.

There is something so special about diamonds, and they are so valuable, that people have always been prepared to go to the most extraordinary lengths to find them. But how would we feel about the uniqueness of diamonds if it was possible to make one in a laboratory, just like the real thing, down to the nearest atom? In the last few years there has been a scientific race to do exactly this: to manufacture the perfect gem diamond.

Hidden deep beneath the Earth's surface lie one of the most destructive and yet least-understood natural phenomena in the world - supervolcanoes. Only a handful exist in the world but when one erupts it will be unlike any volcano we have ever witnessed. The explosion will be heard around the world. The sky will darken, black rain will fall, and the Earth will be plunged into the equivalent of a nuclear winter.

When and how did space and time begin? The birth of the Universe is one of the biggest mysteries in astronomy. It has perplexed the best scientific minds for centuries. Decades before space travel was possible, astronomers dreamed of putting a telescope into orbit to try and answer these fundamental questions. It wasn't until the 1970s, when space flight had become a reality, that NASA resolved to build just such a space telescope. They named it Hubble.

What happens when a completely healthy person wants their leg amputated?

Some people see GM food as a ground-breaking scientific idea that could help to end world hunger and reduce global pollution. Others see it purely as 'Frankenstein foods' on 21st century menus, bringing health and environmental disasters. But what are the real scientific facts behind the newspaper headlines?

If extra-terrestrials do exist they must have a home. Horizon tells the story of the race to find out where in the Universe this might be. The answer, for scientists across the world, lies in the hunt for planets around distant stars.

A handful of children around the world cannot tolerate the sun. Any exposure leads rapidly to skin cancer. They must either play indoors during daylight or be protected from head to toe in UV-proof suits. These children suffer from a strange and rare genetically-inherited disease, xeroderma pigmentosum, or XP, which means that within seconds of the sun's rays touching their skin, they are in danger of developing skin cancer.

Scattered across the world’s oceans are a handful of rare geological time-bombs. Once unleashed they create an extraordinary phenomenon, a gigantic tidal wave, far bigger than any normal tsunami, able to cross oceans and ravage countries on the other side of the world. Only recently have scientists realised the next episode is likely to begin at the Canary Islands, off North Africa, where a wall of water will one day be created which will race across the entire Atlantic ocean at the speed of a jet airliner to devastate the east coast of the United States. America will have been struck by a mega-tsunami.

Conjoined twins are among the rarest of human beings. There are probably fewer than a dozen adult pairs living in the world today. Only a few hundred pairs of conjoined twins are born in the whole world each year - they appear about once in every 100,000 births - but more than half of them are stillborn, and one in three live for only a few days.

It sometimes seems as if our planet has no secrets left - but deep beneath the great Antarctic ice sheet scientists have made an astonishing discovery. They’ve found one of the largest lakes in the world. It’s very existence defies belief. Scientists are desperate to get into the lake because its extreme environment may be home to unique flora and fauna, never seen before, and NASA are excited by what it could teach us about extraterrestrial life. But 4 kilometres of ice stand between the lake and the surface, and breaking this seal without contaminating the most pristine body of water on the planet is possibly one of the greatest challenges science faces in the 21st century.

On August 2nd 1947, a British civilian version of the wartime Lancaster bomber took off from Buenos Aires airport on a scheduled flight to Santiago. There were 5 crew and 6 passengers on board the plane - named "Stardust". But Stardust never made it to Santiago. Instead it vanished when it was apparently just a few minutes from touchdown. One final strange Morse code radio message - "STENDEC" - was sent, but after that nothing more was heard from the plane. Despite a massive search of the Andes mountains no trace of the plane was ever found. For 53 years the families of those who disappeared have not known what happened to their loved ones. But earlier this year the plane suddenly reappeared on a glacier high up in the Andes, more than 50 km’s from the area where the plane was last reported.

In the summer of 2000, one of the great frontier cities of the Roman Empire, the city of Zeugma, all but disappeared from the face of the Earth under the flood waters of a dam. In a bid to modernise, the Turkish government has embarked on one of the most ambitious engineering projects in the world, building a series of dams on the Euphrates over the past twenty years. Almost every dam threatens ancient remains that lie below in one of the most archaeologically rich regions of the world. The completion of the Birecik dam, featured in this film, has flooded the valley where Zeugma is buried. The city on the flat plain has entirely disappeared and the waters have now risen to cover 30% of the city on the hillside.

At the heart of the AIDS epidemic in Africa, there is a deadly mystery that has puzzled scientists for years. There are groups of people who are four times less likely to get HIV than other people, sometimes living just yards away, across a single valley - people with apparently similar behaviour and lifestyle. Scientists realised that if they could understand why these people are so much less vulnerable to the HIV virus, it might lead to an answer that could save millions of lives. And after 15 years of detective work it turns out there may be a remarkably simple answer: the high risk areas for HIV coincide with tribes who are uncircumcised. In Africa, it seems a man is much more likely to get HIV if he is uncircumcised.

Amazing new discoveries in South America are revolutionising what we thought we knew about the dinosaur world. It now seems that South America was home to both the largest meat-eater - so new it's still without a name - and the largest herbivore - the enormous long-necked Argentinasaurus. And what's more, these dinosaurs lived at the same time in the same place. So it's possible that like in a science fiction movie, in this prehistoric world these two giants of their kind fought each other in a spectacular clash of the Titans.

In June 2000, astronomers made an extraordinary discovery. One that promises to solve one of the biggest problems in cosmology - how and why galaxies are created. Incredibly, the answer involves the most weird, destructive and terrifying objects in the Universe - supermassive black holes. Scientists are beginning to believe that these forces of pure destruction actually help trigger the birth of galaxies and therefore are at the heart of the creation of stars, planets and all life. Supermassive black holes are so extraordinary that until recently, many people doubted that they existed at all. The idea of giant black holes the size of the Solar System seemed more like science fiction that reality - such monsters would be so powerful that they could destroy the very fabric of the Universe. But in the last five years a series of discoveries has changed our understanding of supermassive black holes and galaxies forever. Using the powerful Hubble Space Telescope, scientists have been scanning nearby galaxies, searching for these giant black holes. It's a difficult job - by their very nature black holes swallow light - so can never be seen. So what scientists have been looking for is the effect of their massive gravity, hurling stars around them at immense speed. What they've found is more extraordinary than anyone could ever have imagined; not just evidence that these vast destructive monsters exist… but so far they're in every single galaxy toward which they have turned their telescopes. These giant agents of destruction appear to be common throughout the Universe. Scientists now think supermassive black holes are a fundamental part of what a galaxy actually is. Lurking at the heart of every single galaxy is a giant black hole of apocalyptic proportions - and that includes our own galaxy, the Milky Way. Astronomer Andrea Ghez has been studying the heart of the Milky Way for the last five years. What she's discovered is irrefutable evidence for a giant black hole, 3 billion times the size of our own sun. A black hole that could destroy the entire Solar System. And as Horizon was filming in July 2000, Ghez got some terrifying images - of the giant monster sucking up gas and stars at the galaxy heart. So what is this giant monster doing at the heart of our galaxy? What effect will this giant black hole 25,000 light years away have on us and the rest of the galaxy around it? These are questions that have been puzzling astronomers for the last few years - and in June, two separate groups of scientists found evidence that points to a startling answer. Rather than being destructive parasites, it seems that supermassive black holes may be essential in the very creation of the galaxies they live in. Exactly how our galaxy was created has mystified astronomers and physicists for years. Although there have been many theories, there's little evidence to explain how the gas in the early Universe condensed to form the galaxy we see today. Now scientists realise they've been missing a vital ingredient - a supermassive black hole. The immense gravity of a giant black hole might trigger the gas to collapse in the first place. By churning up the gas around it, a giant black hole would trigger the birth of stars, planets and life itself. Despite being the most destructive thing in the Universe, scientists now think our supermassive black hole could be crucial in creating the galaxy as we know it. The supermassive black hole in our own galaxy may be the reason we exist, but recent work suggests it may also be our end. At present Earth is so far away from the black hole that it can't affect us, but physicist John Dubinski thinks all that could change. In January 2000 he graphically simulated the final fate of our galaxy. In 3 billion years we will collide with the next door galaxy, Andromeda. The resulting apocalypse will force the Earth and our Solar System out of orbit. Dubinski has calculated a worrying 50:50 chance that we'll be sent hurtling in towards the black hole at the centre of this maelstrom. This would be fatal for the Earth.

In 1965 in the Canadian town of Winnipeg, Janet Reimer gave birth to twin boys - Bruce and Brian. Six months later a bungled circumcision left Bruce without a penis. Based on a radical new theory of gender development the decision was taken to raise Bruce as a girl. In 1967 Bruce became Brenda and for the next three decades this case would be at the heart of one of the most controversial theories in the history of science.

Horizon puts Graham Hancock's controversial theories about the past to the test, dissecting his evidence for a lost civilisation.