Two thousand years ago a mysterious and little known civilization ruled the northern coast of Peru. Its people were called the Moche.
They built huge and bizarre pyramids that still dominate the surrounding countryside; some well over a hundred feet tall. Many are so heavily eroded they look like natural hills; only close up can you see they are made up of millions of mud bricks.
Several of the pyramids, known as 'huacas', meaning sacred site in the local Indian dialect, contain rich collections of murals depicting both secular and sacred scenes from the Moche world. Others house the elaborate tombs of Moche leaders.
Out in the desert, archaeologists have also found the 2,000-year-old remains of an extensive system of mud brick aqueducts which enabled the Moche to tame their desert environment. Many are still in use today. Indeed there are signs that the Moche irrigated a larger area of land than farmers in Peru do now.
But who were the Moche? How did they create such an apparently successful civilisation in the middle of the desert, what kind of a society was it, and why did it disappear? For decades it was one of the greatest archaeological riddles in South America. But now at last, scientists are beginning to come up with answers.
As archaeologists have excavated at Moche sites they've unearthed some of the most fabulous pottery and jewellery ever to emerge from an ancient civilization. The Moche were pioneers of metal working techniques like gilding and early forms of soldering. These skills enabled them to create extraordinarily intricate artefacts; ear studs and necklaces, nose rings and helmets, many heavily inlaid with gold and precious stones.
But it was the pottery that gave the archaeologists their first real insight into Moche life. The Moche left no written record but they did leave a fabulous account of their life and times in paintings on pots and vessels. Many show everyday events and objects such as people, fish, birds and other animals. Others show scenes from what, at first sight, look like a series of battles.
But as the archaeologists studied them more closely they realised they weren't ordinary battles; all the soldiers were dressed alike, the same images were repeated time and again. When the battle was won, the vanquished were ritually sacrificed; their throats cut, the blood drained into a cup and the cup drunk by a God-like deity. It was, the archaeologists slowly realised, a story not of war but ritual combat followed by human sacrifice.
But what did it mean. Was it a real or mythological scene; and, above all, was it a clue to the Moche's life and times?
The first break through came when a Canadian archaeologist called Dr Steve Bourget, of the University of Texas in Austin, discovered a collection of bones at one of the most important Moche huacas. Examining them he realised he wasn't looking at an ordinary burial site. The bodies had been systematically dismembered and marks on neck vertebrae indicated they had had their throats cut. Here was physical proof that the images of combat and sacrifice on the pots were depicting not a mythological scene but a real one.
Many of the skeletons were deeply encased in mud which meant the burials had to have taken place in the rain. Yet in this part of Peru it almost never rains. Bourget realised there had to be a deliberate connection between the rain and the sacrifices. It lead him to a new insight into the Moche world. The Moche, like most desert societies, had practiced a form of ritual designed to celebrate or encourage rain. The sacrifices were about making an unpredictable world more predictable. A harsh environment had moulded a harsh civilisation with an elaborate set of rituals designed to ensure its survival.
These discoveries answered one question – what was the iconography all about – but still left a central riddle. What had gone wrong; why had Moche society finally collapsed?
The next clue was to come from hundreds of miles away in the Andes mountains. Here climate researcher Dr Lonnie Thompson, of Ohio State University, was gathering evidence of the region's climatic history using ice cores drilled in glaciers.
Almost immediately Thompson and his team noticed something intriguing. The historic records showed that over the last one hundred years, every time the ice cores showed drought in the mountains, it corresponded to a particular kind of wet weather on the coast, a weather system known as an El Nino. In other words drought in the mountains meant an El Nino on the coast. If Thompson could trace back the climate record in the mountains he'd also get a picture of what happened on the coast.
The result was fascinating. The climate record suggested that at around 560 to 650 AD – the time the Moche were thought to have collapsed – there had been a 30-year drought in the mountains, followed by 30 years or so of heavy rain and snow.
If the weather on the coast was the opposite, then it suggested a 30-year El Nino - what climatologists call a mega El Nino – starting at around 560 AD, which was followed by a mega drought lasting another 30 years. Such a huge series of climatic extremes would have been enough to kill off an civilization – even a modern one. Here, at last, was a plausible theory for the disappearance of the Moche. But could it be proved?
Archaeologists set out to look for evidence. And it wasn't hard to find. All the huacas are heavily eroded by rain - but scientists couldn't tell if this was recent damage or from the time of the Moche. But then Steve Bourget found evidence of enormous rain damage at a Moche site called Huancaco which he could date. Here new building work had been interrupted and torn apart by torrential rain, and artefacts found in the damaged area dated to almost exactly the period Thompson had predicted there would have been a mega El Nino. Thompson's theory seemed to be stacking up.
Then archaeologists began to find evidence of Thompson's mega drought. They found huge sand dunes which appeared to have drifted in and engulfed a number of Moche settlements around 600 to 650 AD. The story all fitted together. The evidence suggested the Moche had been hit by a doubly whammy: a huge climate disaster had simply wiped them out.
For several years this became the accepted version of events; the riddle of the Moche had been solved.
There was only one problem. In the late 1990s American archaeologist Dr Tom Dillehay revisted some of the more obscure Moche sites and found that the dates didn't match with the climate catastrophe explanation. Many of these settlements were later than 650 AD. Clearly the weather hadn't been the cause of their demise.
He also found something else. Many of the new settlements were quite unlike previous Moche settlements. Instead of huge huacas, the Moche had started building fortresses. They had been at war.
But who with? Searching the site for clues, Dillehays's team were unable to find any non-Moche military artefacts. It could only mean one thing. The Moche had being fighting amongst themselves.
Dillehay now put together a new theory. The Moche had struggled through the climatic disasters but had been fatally weakened. The leadership - which at least in part claimed authority on the basis of being able to determine the weather – had lost its authority and control over its people. Moche villages and and/or clan groups turned on each other in a battle for scare resources like food and land. The Moche replaced ritual battles and human sacrifices with civil war. Gradually they fought themselves into the grave.
Yet even that's not the whole story. Today, along the coast of Peru it's impossible to escape the legacy of this lost civilization. Their art lives on in the work of local craftsmen. And if you travel to the highlands, the Moche tradition of ritualised combat is preserved in the Tinku ceremonies where highland villages conduct ceremonial battles against each other in the hope of ensuring a good harvest.
Today, after 1,500 years, the Moche, and their legacy are beginning to take their place in world history. The story of the Moche is an epic account of society that thought it could control the world and what happened to it when it found it couldn't. It's a story of human achievement and natural disaster, human sacrifice and war.
Moche Art and Archaeology in Ancient Peru, edited by Joanne Pillsbury, 2001 (National Gallery of Art, Washington)
Ritual Sacrifice in Ancient Peru, edited by Elizabeth P. Benson and Anita G. Cook, 2001 (University of Texas Press)
Royal Tombs of Sipan, by Walter Alva and Christopher B. Donnan, 1995 (University of California Press)
Moche Portraits from Ancient Peru, by Christopher B. Donnan, 2004 (University of Texas Press)
Splendors of the Moche. In National Geographic, vol 177, no.6, June 1990
Tales from a Peruvian Crypt. In Natural History, vol 103, no.5, May 1994 by Walter Alva and Christopher B Donnan
Moche Politics, Religion and Warfare. In Journal of World Prehistory, vol 16, no 2, June 2002, by Jeffrey Quilter
Punctuated equilibrium: Searching the Ancient Record for El Nino. In The Quarterly Review of Archaeology vol 8, no.3, 1987 by Michael Moseley