Watched hit Netflix show Ancient Apocalypse yet? Spread across eight episodes, controversial journalist Graham Hancock visits an array of ancient sites, and attempts to prove an outlandish theory – that an advanced Ice Age civilisation once roamed the Earth, spreading ancient wisdom to hunter-gatherers, before being wiped out in a great flood. Hancock suggests the sites featured in the show are far older than they appear and contain allusions to this advanced people, but most archaeologists are highly critical of his views, dismissing them as unfounded conspiracy theories. That's not to say the sites aren't genuinely mysterious – these are the real-life secrets hidden in the series' sites...

Episode 1 takes us to Indonesia, where Gunung Padang – the largest megalithic structure in southeast Asia – is found on the main island of Java. Its name means 'mountain of light' or 'mountain of enlightenment', and pilgrims often purify themselves at an ancient spring before ascending the 360-foot (109m) hill. While some archaeologists have dated the hilltop structure to the Bronze Age (2500-1500 BC), its exact date is still very much in question.

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Thousands of odd-shaped hexagonal stone slabs are strewn across the top of the hill. These naturally-occurring blocks are called columnar jointing and form when volcanic (basalt) rock cools and cracks into distinctive shapes. But some archaeologists have controversially argued that they were carved by humans and used as building materials to construct an incredible stepped pyramid.

The argument holds that the site is the oldest pyramid on Earth – older even than those in Egypt – and that carbon dating suggests subterranean parts of the structure date from as far back as 28000 BC. Most experts discredit the theory, pointing out that Ice Age-era hunter-gatherers did not possess the capacity to build something of such scale, and that the site's location on top of an extinct volcano could have muddied the dating process.

Also in Episode 1, we visit Nan Madol on the Micronesian island of Pohnpei. It's the only surviving ancient city built on a coral reef, and nobody knows how it was constructed. With no carvings or art left to decipher, there’s just the legends and histories of the Saudeleur people who ruled here for 500 years. It’s widely believed that the city was built between AD 1200 and 1500.

Like at Gunung Padang, the structures rest partly on naturally occurring hexagonal and pentagonal basalt rock columns formed from coral rock. The outer walls are still impressively intact, supporting elevated platforms that were topped with traditional thatched structures. One elaborate building once housed the royal mortuary where kings' bodies were placed (though they were later buried elsewhere).

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Nobody knows how they raised the walls and Pohnpeian tradition holds that they were flown in by magic. After Nan Madol was abandoned, religious ceremonies were occasionally held here until the late 1800s. When Nan Madol was erected Pohnpei’s population was less than 30,000 people, so it must have required a huge community effort to build it.

In Episode 2 we visit the Great Pyramid of Cholula – by volume, the largest pyramid on Earth. Estimates hold that it once rose to 213 feet (65m) and was built with mud, straw and adobe bricks. Already overgrown by grass and shrub when the Spanish first arrived in Central America, the pyramid's history is still shrouded in mystery, and according to myth it was constructed by a giant.

Cholula’s pyramid was completed around AD 1200 and dedicated to Aztec founder god Quetzalcoatl. But years later, when archaeologists cut tunnels through the body of the structure they discovered a series of increasingly older pyramids, like the layers of a Russian doll. The oldest dates to around 300 BC and is 56 feet (17m) high, while archaeologists acknowledge that the site could be even older.

The pyramid was built over an important spring perhaps representing passage into the underworld, solidifying Cholula as a sacred and ceremonial space. Excavations have revealed a network of passageways totalling five miles (8km) in length, while seashells, altars, ceremonial figures and the burial places of human sacrifices have all been found within.

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Built around AD 650-700 and abandoned by AD 900, Xochicalco was a true melting pot of Mesoamerican cultures, and the product of Aztec, Olmec and Zapotec influences. It enjoyed a two-century period of political and commercial dominance after traditional power centres like Tikal, Palenque and Teotihuacan had broken down as part of a massive cultural regrouping.

The city is exceptionally well-preserved. Split into three distinct areas, there are plazas, sacred precincts and an underground observatory, while the large ball court is thought to be the oldest of its kind in central Mexico. One pyramid is dedicated to the rain god, Tlaloc, and another to the feathered serpent god Quetzalcoatl (or Kukulkan).

Carved glyphs include a unique mix of Aztec and Mayan symbols and numbers, while we know the city was in contact with the Mixtec Oaxaca and the Zapotec Indigenous peoples. In the series Hancock focuses on one particular glyph, which he interprets as Quetzalcoatl fleeing a burning island and arriving as a founder of Mesoamerican civilisation. He suggests that the scene refers to an ancient catastrophe that predates the temple.

In Episode 3 we head to the Mediterranean island of Malta, where these so-called 'cart ruts' can be found near Dingli cliffs. Misrah Ghar il-Kbir is the name given to these prehistoric pairs of parallel grooves, but they're nicknamed ‘Clapham Junction’ thanks to their supposed resemblance to the notoriously busy London railway station. The tracks run through solid rock (not soft clay, which adds to the mystery) and are thought to be around 2,700 years old, though we don’t know whether they’re natural or manmade, nor their purpose.

The cart ruts don’t lead to any obvious destinations, nor is there much other datable evidence. They’re an odd mix of straight, curved, short and long, and run along flat and steep surfaces – and through tombs which weren’t there when the ruts were built. Intriguingly, some of the tracks run straight off the edges of ridges and cliffs, and while there are comparable sites around the world (in Portugal, Azerbaijan and India among others), Malta has the highest number.

Because some run into the sea, their uncertain origins have led Hancock to speculate that they led to now-long-lost structures, when this part of the sea floor was above water over 12,000 years ago. The short answer is: no one knows.

Another ancient Maltese site visited by the series is the Mnajdra temple complex on the southern coast – said to date back to 3600 BC and consisting of east, south and central temple buildings. A decorative doorway in the south temple exactly marks the position of the sun at the spring and autumn equinoxes, and people from all over the world flock to witness the twice-yearly event.

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During the equinox, the sun beams directly onto one of the two large hole-ridden slabs inside. Some suggest that the holes represent birth, and that the equinox at the temple constituted some sort of fertility ritual. There are many more megalithic sites across the world with similar alignments, ranging from Stonehenge in England to the Temple of Karnak in Egypt.

Together with five other temple complexes – Ġgantija, Ħaġar Qim, Skorba, Ta’ Ħaġrat and Tarxien – Mnajdra is part of the Megalithic Temples of Malta UNESCO World Heritage Site, and all six rank among the oldest freestanding stone buildings on Earth. It goes without saying that there is much we don't know about these ultra-ancient marvels, but we do know that their construction required remarkable technical skill.

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Episode 4 delves into the Bahamas – or to be more accurate, the ocean nearby. Bimini is separated from America's mainland by a deepwater channel known as the Straits of Florida, and on the seafloor lies a set of giant stones known as the 'Bimini Road' or 'Bimini Wall'. The stones were discovered in 1968, and no one is 100% sure whether they're a natural formation or part of an ancient harbour.

Because the slabs are uniform in shape and there are regular lines between the rocks, some speculate that they were made by human hands, but archaeologists believe the formation is likely a stretch of fractured beach rock formed 3,000 years ago by natural processes. As impressive as they are, the ocean-bed slabs are probably not man-made.

That hasn't stopped some from claiming it could be part of the lost city of Atlantis. In the episode, Hancock looks at a 16th-century map which he believes details this ancient harbour. The place is an excellent scuba site, so why not take the plunge and decide for yourself...

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Gobekli Tepe is a gobsmacking feat of ancient engineering. In Episode 5 we visit the site, located on a hilltop in southeast Turkey, where more than 20 large circular stone structures were carved from the natural limestone found nearby. Built between 9600 and 8200 BC – a solid five millennia before Stonehenge – the site also includes intricate carvings of humans and animals. It's pretty astounding to think that the hunter-gatherers that built this site did so without access to the wheel, metal tools, fire or domesticated animals.

Researchers are still trying to understand exactly why this pre-Neolithic site was constructed, and until 1994 it was thought to be an abandoned medieval burial ground. We know there was probably a permanent settlement here, with excavations revealing houses and grinding tools. The site's extraordinary age has led to all sorts of speculation; some even suggest it's the site of the biblical Garden of Eden.

In Enclosure D, Pillar 43 (the ‘Vulture Stone’) depicts animals, insects and a human figure. Some say the scene represents death, others that it's to do with the stars, while one controversial theory holds that it commemorates a devastating comet strike, known as the Younger Dryas impact hypothesis. Another unsolved mystery is whether the site was intentionally buried, as some scientists think it's unlikely this could have occurred naturally.

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Roughly the same age as Gobekli Tepe, Karahan Tepe (Keçilitepe) was only discovered in 1997. A survey in 2000 recorded chisels, beads, stone pot fragments, arrowheads and blades here, suggesting that the site's residents survived mostly through hunter-gathering or animal husbandry, unlike other Neolithic communities that relied on agriculture.

Later studies revealed similarities with sister site Gobekli Tepe, from its T-shaped pillars to its animal reliefs depicting snakes, rabbits and gazelles. There are also major differences: at Karahan Tepe, the main chamber’s carved edge may have served as a ritual gathering space, while a side chamber features 11 phallus-shaped pillars overlooked by a sculpted human head with a serpent body.

So far two chambers have been excavated, although ground-penetrating radar technology suggests there are many more to be explored. Probably settled by hunter-gatherers, the site goes against mainstream archaeological wisdom that hunter-gatherers settled as farmers before they built large, permanent structures.

In Episode 6, we visit Serpent Mound, around 70 miles (113km) east of Cincinnati. This elongated, snake-like formation is found in the ground in a forest clearing in Adams County, and is believed to have been constructed by Native Ohioans. It's the world’s largest serpent effigy, and today it forms the bulk of the Serpent Mound Historical Site.

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The mound has an oval-shaped head and a meandering body that's 1,348 feet (411m) long, 20-25 feet (6-7.6m) wide and five feet (1.5m) tall at its highest point. Nearby are three burial mounds – two built by the Adena culture (800 BC to AD 100) and one by the Fort Ancient culture (AD 1000 to 1650). The mound sits in a quiet park also home to a historic village and other burial sites.

Though not for lack of trying, archaeologists haven’t been able to pinpoint a specific start date for the mound, with estimates varying wildly from AD 1070 to 321 BC, a date controversially proposed by a study in 2014 that used carbon dating. In the Netflix episode, Hancock insists that the site may be far older – another heavily contested claim.

In Episode 7, we visit Turkey's Cappadocia region, which is dotted with more than one hundred underground cities. Multi-chambered Derinkuyu is the largest; the sprawling subterranean commune was built to protect its inhabitants from invading forces during sieges. Capable of housing up to 20,000 people, the hidden city’s name means ‘deep well’, and the rooms were carved out of the rock by hand axes.

Some observers – including Hancock – reckon the city was designed for everyday living, not just as a refuge from attack. After all, the network housed food stores, stables and wine-making facilities, and had 15,000 air ducts connecting the upper levels to the surface. It's still contested who first built Derinkuyu and why; some think it was the Phrygians around 800 BC, but the earliest written reference to the site is not until 370 BC. We also know that Christians expanded the tunnels and added chapels in the 7th century AD.

Some 17 miles (27km) away lies Kaymakli (pictured), another underground city thought to connect to Derinkuyu via a series of (no longer accessible) tunnels. These days tourists must visit them separately, but it's not uncommon for underground cities in Cappadocia to be connected to one another in this way.

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