Our ancestors built many amazing cities and temples and carved so much fascinating artwork, but often the methods and reasons behind what they did have been lost in the mists of time. These pictures reveal the breathtaking work of the ancient world, and also show that thousands of years on there remain more questions than answers...

Are these megaliths the petrified remains of a Roman legion turned to stone by the wizard Merlin? So local legend would have us believe, but the truth could be just as fascinating. Near the village of Carnac in Brittany, northern France, a vast area of 100 acres (40ha) holds around 3,000 standing stones marching into the distance as far as the eye can see. Some stones are placed in regimented lines, others in circles and some on top of another to form tombs.

These stone armies were the work of Neolithic people dating from between 3,000 to 4,500 BC. It’s an amazing testament to the ingenuity of these pre-Celtic people that they could hew the granite by hand, drag the stones up to 30 miles (50km) and place them so accurately. Even more surprising when you consider the megaliths vary in size, with the tallest around 21 feet (6.5m).

So why did these early people go to so much trouble? Historians and scientists have long puzzled over the purpose of these stones. One researcher claims they were an earthquake detection device, while others say they are temples to honour the ancestors, or some kind of calendar or astrological device. Of course, their use could have changed over the thousand years they were arranged, so perhaps we will never know for sure. What is certain is that they are an awe-inspiring sight.

In 1860, French explorer Henri Mouhot was hacking his way through the Cambodian jungle when he suddenly came upon a magnificent temple hidden among the trees. Only a few Buddhist priests and farmers lived there and they knew nothing of its history. Over the years, historians have pieced together the story of a great empire which dominated the area from around AD 800 to 1400. So why was Angkor Wat, located near modern Siem Reap, left to decline in the 15th century?

We know the temple was only a small part of a giant city, and it was populated by Hindu native people, the Khmer. The main temple was linked by cloisters and stairways, and decorated with images of battles and legends written in Sanskrit. These legends tell of mighty kings, revered as gods, who built these enormous structures. Satellite imagery also shows that the rest of the city, now under the jungle, contained hundreds of simpler houses, paddy fields and interconnecting canals.

The city's demise was swift. In the 15th century, battles with the Vietnamese and Thai people proved devastating for the Khmer. But other factors may have come into play too. Did a switch from Hinduism to Buddhism mean the Khmer stopped believing the king was a god, thus causing a lack of unity? Did they cut down too much jungle and cause the paddy fields to silt up and so denude their food resource? We don't know but, by 1450, Angkor Wat had almost become a ghost town.

The great pyramids are 4,500 years old while Stone Henge is 500 years older. Impressive – but imagine a city that is 7,000 years older than that! Gobekli Tepe, in modern-day Turkey, was built 12,000 years ago, around the end of the last Ice Age. This fact breaks all the rules that humans had to be settled farmers in order to create cities. Gobekli Tepe was built so early in human endeavour that it has led some to speculate that this could even be the site of the Garden of Eden.

While mammoths and sabre-toothed tigers still roamed the earth, these prehistoric people were constructing elaborate temples decorated with beautiful carvings. Located high on a ridge in southeastern Turkey, the site is huge with only a small part excavated so far. There are 43 monolithic limestone pillars with stone walls possibly forming temples. The area was probably forested and temperate in those days so people lived as hunter gatherers.

This amazing site was built before the use of the wheel, cloth, metal tools, fire, pottery and domesticated animals. So could it be the Garden of Eden, as mentioned in the Book of Genesis? The Bible makes no mention of built structures in the Garden and it claims Adam and Eve were banished and were not allowed to return so could not have built anything later. Nevertheless, the extreme age of Gobekli Tepe must make it the oldest human site yet discovered.

We all know what pyramids should look like – a large square base with straight lines rising up to a perfect point. So what do we make of this early Egyptian pyramid that slopes two-thirds of the way towards the top? Built during the reign of the pharaoh Snefru, the Bent Pyramid gives us vital clues as to how these ancient tombs were first constructed. It has a square base and is about 345 feet (105m) tall, with two internal compartments connected by a narrow corridor.

 The outside view shows the lower part of the pyramid was constructed in the usual style but something happened to make the builders abruptly change the angle of the slope. Perhaps the sudden death of the person the tomb was built for meant the builders had to finish quickly. Or the collapse of another pyramid being built at the same time made the builders aware of a flaw in the design. Or was this just a prototype, a way of learning valuable lessons on the way to the later pyramid masterpieces?

Hundreds of mysterious free-standing stones carved with enigmatic and stylistic symbols can be found all over Scotland from the Western isles to the east. They date from AD 500 to 900 and no one knows for sure what the symbols represent. Some are clearly animals and birds, real or imagined, while others are everyday objects such as combs or mirrors or objects of unknown meaning – but the same strange symbols are repeated again and again.

The Pictish people, who inhabited the land north of Hadrian’s Wall before the Scots, were as mysterious to history as the stones they carved. They had no writing system that we know of, so we can only guess what the drawings mean. Around 50 symbols come up repeatedly and generally are placed in pairs. There are salmon, deer, geese and fantastical monsters. Objects include a crescent and V rod, a comb and mirror and a double disc with Z rod.

The stones may be markers of territory or they might be tombstones, with the symbols representing married couples or clans. The stones might even be political statements opposing the growth of the new religion of Christianity. Maybe the symbols were some kind of writing that we cannot interpret. Perhaps we will never know their meaning, nor the lives behind their mysterious creators. Now discover the world's most beautiful ancient ruined cities.

Imagine the flat, grassy landscape of central Laos and an ancient area called Xiangkhouang Province, which was most recently ravaged by bombs during the Vietnam War. You will be amazed to see thousands of huge stone jars, some up to 10 feet (3m) tall and three feet (1m) wide scattered randomly across the plain. What are these jars for, who made them and why are they there?

They are thought to date from the Iron Age, around 500 BC to AD 500, and research in the 1930s uncovered human skeletal remains and artefacts including beads, ceramics, iron implements and bronze jewellery, among other relics. The jars were mostly empty with only occasional objects found inside and they possibly once had lids made of perishable materials. Maybe they are funeral urns or memorials to the dead. Or maybe they were used to hold water.

How were these jars, some weighing 30 tonnes, transported from their quarry about five miles (8km) away? Were they carved out at the quarry then rolled to their resting place? This area is still being excavated, but it seems there are many more places which hold these jars – possibly up to 90 sites. We know nothing of the people who created them, yet it was a feat of great organisation, technical skill and immense hard work. The jars remain today a strange and wonderful sight.

Just 30 miles (48km) north of Mexico City lies an abandoned ruined city covering eight square miles (20sq km). Its pyramids and palaces, roads and irrigations systems lie deserted as they did in the 12th century AD, when the Aztecs came upon the place. The Aztecs named it Teotihuacan or ‘the place where one becomes a god’, but we do not know what the people who built the city called it, since they left no written record. They abandoned the place around AD 700 and took their secrets with them.

But if you'd visited Teotihuacan around the year AD 600, you would have found a thriving metropolis of around 200,000 people. The Pyramid of the Sun, the third largest in the world, would have glowed bright red under the sun, and you could have walked the mile-long Avenue of the Dead flanked by temples and palaces. You would also have seen people using sophisticated irrigation systems necessary to feed such a large population.

Just 100 years later, Teotihuacan was deserted. What happened to this amazing civilisation? It’s possible that soil erosion meant that people were no longer able to feed themselves and this could have caused civil war. Perhaps they were attacked by outside tribes. We do know a fire swept through the city and the people left. Although investigations at the Pyramid of the Moon continue, we may never know the name of the city or its people, but what they left behind is spectacular.

In 1974, a farmer was digging on his land in Xi’an in central China. To his amazement, he discovered a few human figures made of terracotta. When the archaeologists arrived, they started uncovering statue after statue. Although work continues to this day, they estimate there is a hidden army of 8,000 soldiers, 130 chariots, 520 horses and 150 cavalry. Each figure was constructed individually with unique faces and physiques, and they carried sophisticated cross bows and swords.

This model army was made for the Emperor Qin Shi Huang who died in 210 BC and who was the first to unite China. His memorial was created in the form of an underground necropolis protected by an army of soldiers and weapons. The complex covers about 38 square miles (98sq km) with the Emperor’s tomb in the centre. However, no one has yet entered the tomb as there are rumours of booby-trapped weapons protecting the entrance and rivers of mercury flowing around the tomb.

The Chinese government has decided it's wise to leave the tomb unopened until they know more about it. The ancient Chinese thought mercury bestowed eternal life and Emperor Qin reportedly drank it every day – probably the cause of his death. If the ground is contaminated with mercury it would be dangerous to enter, not to mention the risk of booby-trapped cross bows. It looks like a case for Indiana Jones perhaps? Love this? Now check out new secrets of the world's ancient wonders revealed.

Every year at dawn for a few days around 21 December, the winter solstice, a narrow beam of sunlight enters a small, perfectly positioned opening at Newgrange Tomb in Ireland. It illuminates the floor of the tomb and as the sun rises gradually penetrates the whole chamber until it is covered in light. It's astonishing to realise how accurately this opening must have been positioned for this to happen – even more so when we realise the Neolithic farmers who did this lived 5,200 years ago.

Newgrange is located in County Meath and consists of a large circular mound 279 feet (85m) in diameter and around 42 feet (13m) high with a 62-foot (19m) stone passageway and chambers inside. The mound is ringed by 97 large kerbstones, some beautifully decorated. It’s now also thought that the structure was not just a tomb, but more like a cathedral – a place for worship and religious ceremonies. The bones of the community’s elite were buried here, illuminated every winter solstice by a beautiful beam of light.

“We could hardly conceive how these islanders, wholly unacquainted with any mechanical power, could raise such stupendous figures,” wrote Captain James Cook in 1774. He was talking about the stone heads, or moai, that litter Easter Island, an isolated volcanic island about 2,000 miles (3,200km) off the coast of Chile. Also called Rapa Nui, the island is only 14 miles (23km) long by seven miles (11km) wide, but has around 900 sculptures, averaging 13 feet (4m) in height.

Research shows Polynesian people arrived on the island between the first and sixth century AD and found a richly wooded landscape. They prospered and began to construct these amazing statutes. It's thought that the monuments were originally brightly painted and had bright coral eyes too. The red top knots were added later. Recent studies show the heads were placed near a source of clean water – perhaps as a thanksgiving. So why did the people turn against their god statues?

By the 18th century, the population of Easter Island was reduced to 600 and the people had turned on the statues and even started to topple them. Could deforestation have led to civil war, starvation and anger against the old gods? Perhaps without trees, the islanders had to spend time providing habitats for the birds they revered, which left no time for building. Whatever the reason, the stupendous monuments remain a wonder. Now see incredible ancient wonders rebuilt before your eyes.

The Great Pyramid of Giza has been so carefully studied that one might think there are no secrets left to be discovered. However new techniques have enabled scientists to penetrate inside the pyramid’s structure in areas which have no outside access. Subatomic particles called muons move more easily through empty space than through solid structures – so by bombarding the ancient pyramids with these particles and tracking them, experts can get a better picture of what lies within. What they've found is amazing.

The pyramid was built around 4,500 years ago and the method of its construction is always up for debate. But in 2017, scientists announced the discovery of a curious void that's at least 100-feet (30m) long hidden deep within the ancient structure – it's similar to the Grand Gallery corridor that leads to pharaoh Khufu's burial chamber. It has led to much speculation about what this space was used for.

One suggestion is that it was an internal ramp used to move the massive blocks of stone to the top of the pyramid. The idea that it might be a hidden tomb containing artefacts is improbable, but we are not likely to know any time soon as the Egyptian authorities will probably never give permission for drilling to access the Great Void. In the meantime, we will just have to use our imaginations. Discover more of Egypt's mysteries here.