A study released by NASA has revealed that modern civilisation is heading for collapse due to economic instability and pressure on the planet's resources.
Today we look at seven civilisations that have disappeared and the reasons behind their demise.
1. Nabta Playa (7000 - 6500 BCE)
This advanced community was situated close to modern-day Cairo. Archaeological discoveries show that they had designed villages, domesticated cattle, and made ceramics. Remnants of animal sacrifices suggest that they played a part in the formation of Ancient Egypt's Hathor Cult. They left behind a circular stone structure, perhaps a prehistoric calendar, making it one of the world's earliest known astronomical devices.
CAUSE: Nobody knows what happened to the people who lived in Nabta Playa. However, some researchers suggest they were responsible for the rise of the Ancient Egyptian civilisation due to their advanced knowledge of agriculture, astronomy and mathematics.
2. Indus Valley Civilisation (3300 - 1900 BCE)
Regarded as one of the greatest civilisations of the ancient world, the Indus Valley or Harappan Civilisation covered over one million kilometres of land in what is now Afghanistan, Pakistan and part of India. It contained many cities and villages made of baked mud bricks, including the famous Mohenjo Daro.
The people had a well-developed writing system and were skilled in engineering, mathematics, agriculture, and even had their own drainage and plumbing systems that included indoor toilets. They were also the first civilisation to produce cotton cloth.
It is estimated that at its peak there were around five million people. By 1700 BCE most of the cities were abandoned.
CAUSE: Forgotten until the 1920s, when it was rediscovered by archeologists, possible reasons for its demise include Indo-European invasions and a collapse in agriculture due to climate change.
3. Minoan Civilisation (2700 - 1500 BCE)
Named after the mythic King Minos, the Minoan civilisation lived on the island of Crete and is the first known civilisation in Europe. They had a hieroglyphic and later a more advanced form of writing, pottery, art and craftsmanship. They enjoyed fruit, honey and wine and worshiped female deities and animal gods. Thriving on overseas trade, they were a wealthy civilisation that built complex and extravagant palaces.
CAUSE: While it is unclear exactly what happened, scholars suggest that around 1450 BCE a large natural disaster - probably a volcano - caused extensive damage. However, the Mycenaeans' may have conquered the island.
4. The Mayan Empire (2000 BCE - 900 CE)
The Mayan Empire was a dominant and advanced civilisation which covered a vast landmass in Mexico and Central America. While the first settlement dates to around 1800BCE, the discovery of the Tomb of King Pacal showed the level of complexity and thought that the Mayan Empire was capable of - architecture, written language and art. They invented chocolate and herbal medicine and by 600 AD they had schools, libraries and hospitals. They were a highly religious population, who had developed their own astronomical systems. They built hundreds of cities and at its peak the Mayan population may have been up to two million. By 1000AD Mayans lived in small villages and their great cities were in ruins, buried beneath the rainforest.
CAUSE: Nobody really knows what happened but theories include over population, warfare, overuse of land and drought.
5. The Nabataeans ( 37 - 100 CE)
Literally carving their cities into desert cliffs, the Nabataeans were a literate and diverse group of people who occupied southern Jordan, Canaan and northern Arabia. They are best known for the capital city of Petra, which still exists today and can only be accessed via a 1200 meter long crack in rock. They were skilled engineers who built a complex system of dams and canals that enabled them to thrive in the desert region. Despite evidence that they were able to read and write, no written literature survives. Their population was estimated to be 20,000.
CAUSE: By 106 CE, the Nabataeans were conquered by the Roman Empire and they were reduced to small groups who were eventually absorbed into the Greco-Roman culture.
6. The Khmer Empire (800-1500 CE)
Situated in modern-day Cambodia, the Khmer Empire was one of the most powerful empires in Southeast Asia, spreading across Vietnam, Thailand and Laos. The Empire had an extensive network of agricultural farming and excelled at engineering. They developed a road network that included bridges, building hospitals and elaborate temples. With high stone walls and military might, they fought many wars against the Annamese and Chams.
CAUSE: The demise of the Khmer Empire was due to a number of factors, including the arrival of Theravada Buddhism which challenged the government, increasing invaders, overuse of the land and deaths from the Black Death.
7. The Inca Empire(1200-1572 CE)
The largest empire ever to exist in the Americas, the Inca carved a system of roads into rugged mountains that stretched around three times the diameter of the earth. They built complex cities and were regarded for their cleanliness and peacefulness. While they did not have a written language, they did have a system for recording information such as census data. They were a deeply religious society, with many nature gods.
An archaeological discovery in 1999 found the mummies of three children who had been left as sacrifices at a shrine. They were adept at agriculture, stone-work and were well-known for their complex, multi-coloured textiles. Estimates of the Inca population at its peak are varied, but range from 4 million to more than 37 million.
CAUSE: The Inca were conquered by the Spanish. The arrival of the Spanish was catastrophic because they brought new diseases and engaged in repeated warfare. The population declined at a ratio of 58:1 in the years 1520-1571.