Britain boasts a past that’s far stranger than might be expected. Those who look far enough back can discover cannibals, shamans and sun, moon and water-worshippers, Christian initiations and pagan bog sacrifices. There’s also evidence of prehistoric mummification, 20th-century magic and all manner of sites that blend old and new beliefs into complex, compelling hybrids. Maybe it’s because so many different peoples have shaped this island, making their marks on the landscape, on artefacts and on the next generation. Each generation, each wave of immigrants, each trade network has brought something new to the party.

Research often focuses on how such wonders were wrought – and modern scientific techniques have answered many pressing questions. But my book Secret Britain, aims to explore the more elusive question: why? Definitive answers aren’t always easy to come by, but we can stand puzzled, moved or uplifted in the presence of this strange and secret history. It’s exactly what the ancestors were doing, too.

Cheddar Gorge, Somerset: paleolithic, 14700BC

Around 13,000 years ago, Britain was part of continental Europe and still in the throes of the ice age. But the weather was occasionally clement enough to allow bands of hunter-gatherers to walk north and west and make the most of seasonal hunting. They ate horse, hare and reindeer. They also ate each other. The indisputable evidence for cannibalism comes from Cheddar Gorge, a rift of towering limestone cliffs wormholed with natural caves. In Gough’s Cave, bones from six individuals – a toddler, two teenagers and three adults – have cut marks and human bite marks. This isn’t evidence of a hunting trip gone wrong, survivors driven to desperate measures; skulls have been carefully chipped away to form bowls, and arms show delicate zigzag engravings. We don’t know if Cheddar Gorge was specially selected for these eating rituals, or if cannibalism was a normal part of life (and death) in ice-age Somerset. The show caves are currently closed, but the wider landscape – timeless and strange – beckons.

Bryn Celli Ddu, Anglesey: mesolithic and neolithic, from 6000BC

This masterpiece of neolithic architecture on Anglesey was built around 3000BC. The chambered tomb is perfectly aligned so the midsummer sunrise shines along the passage and illuminates the interior. Outside the tomb there’s a “pattern stone” carved with zigzagging contours, a rare decoration more commonly found in tombs in Ireland and France. Was this place built by immigrants settling the area? By locals who’d travelled overseas and adopted new styles? Whoever it was, they chose a place that had already been important for thousands of years. Evidence for ritual on this site goes back to the mesolithic era – the middle stone age – around 6000BC, when hunter-gatherers erected a series of pinewood posts aligned along the same axis as the later tomb entrance. It’s simply staggering to think people have been welcoming the sun in this special place for more than 8,000 years.

Tomnaverie recumbent stone circle, Aberdeenshire: late neolithic, 2500BC

Just east of the Cairngorm mountains there’s a wide hilltop with an unusual type of prehistoric stone circle found only in north-east Scotland. The essential feature is a massive stone laid flat, the “recumbent”, tightly flanked by two standing stones, and a circle of other stones that descend in height order. Is the recumbent an altar table? A frame for viewing celestial phenomena? Or is it a blocked gateway, either to keep things in the circle or keep them out? Many of the stones have inclusions of sparkly quartz, and it seems likely that this monument came to life at night, perhaps representing the cycle of life and death, and dark and light.

Ilkley Moor, West Yorkshire: neolithic and bronze age, 4000-1500BC

British rock art isn’t impressive like the showy runs of bison stampeding across cave walls in Lascaux or Altamira. It’s small, nude, abstract and often quite hard to spot. But Ilkley Moor’s Badger Stone and Barmishaw Stone, show why people get obsessed. The most frequent motif is the cup mark, a small circular depression carved out of the rock face. You’ll also see the cup-and-ring, and multiple cups and rings joined by snaking grooves and gutters. The rock art doesn’t have obvious meaning – the designs don’t appear to be star maps or stories. But maybe we’re reading them wrong. Or maybe they aren’t for “reading” at all but for “doing” – used to make offerings or for fortune-telling. We’ve identified about 7,000 rock art panels in Britain so far, and more are being found every year.

Tintagel, Cornwall: post-Roman British, AD450-650

Tintagel, on the north coast of Cornwall, is commonly associated with King Arthur. Its story actually begins around 450AD, when it became a trading centre with the eastern Mediterranean, taking advantage of the power vacuum left by the Brexiting Romans. Excavations have revealed amphorae containing olive oil and wine from Turkey, Greece, Tunisia and Syria, and fancy foreign tableware. It seems likely this was a court for the kings of Devon and Cornwall. A footprint carved into the rocks at one of Tintagel’s highest points suggests that it may even have been used for royal inauguration – perhaps planting seeds for later stories of a legendary king and a round table of loyal warriors.

Aberlemno stones, Angus: Pictish, AD600-700

The Romans coined the term Picti (“painted people”) as a racial slur for the ferocious guerrilla fighters they encountered in northern Scotland, and at some point the Picts decided to claim the name as their own. We have few reliable historical records of the Picts, but we do have pieces of their art, weaponry and a set of intriguing and as-yet-undeciphered carved standing stones that date from around AD600. In Aberlemno village, these remarkable treasures are around every corner: enigmatic pairs of symbols – the double-disc and Z-rod; the mirror and comb - and identifiable animals, such as fish and snakes. Are these billboards spelling out royal names or lineages? Are they prayers? Memorials? In Aberlemno churchyard, an enormous carved stone slab combines the newly imported Christian cross symbol with what might be a record of the great Battle of Dun Nechtain, in 685, and symbols that might record a name. We just don’t know who.

Gosforth Cross, Cumbria: Anglo-Viking, AD900

This slim column of sandstone, 4.5 metres high, was shaped into a cross in the early 900s. All its surfaces are carved, with interlocking knots, fantastical animals and Viking gods including Odin, Thor and Heimdallr. But this is definitely a Christian cross, probably commissioned by Norse settlers who had arrived in north-west England from Ireland, the Western Isles or Scandinavia. Were they trying to evangelise for the new religion? Were the images of the old gods merely decorative? Or is this a creative attempt to put Christ into a Viking ideology? One image, familiar in Christian art as Christ on the cross with two attendants, is particularly intriguing. Because the attendants aren’t Mary and St John – here, they appear to be Valkyries, the otherworldly women who took fallen warriors to Valhalla.

Dartmoor, Devon: 1300s

We think of Dartmoor as a wild and windswept place, more Mother Nature than human nature. But it’s packed with 20,000 archaeological sites. By 1700BC, during the bronze age, the land had been deforested, and by 1000BC, the weather had got cooler and wetter and people gave up trying to farm the uplands. Some places eventually drew the hardy and desperate back. Hound Tor had been settled and then deserted in the bronze age, but people returned in the 13th century as warmer weather and population pressure pushed peasant farmers on to marginal land. The cataclysm of the Black Death (which is estimated to have killed more than 1.5 million people in Britain) as well as the onset of colder, wetter weather, pushed them back again. The remains of the longhouses are quiet memorial to a village that faced – and ultimately didn’t survive – contagion and climate change.

Glastonbury Tor, Somerset: 1530s

The majestic hill is natural; its strange terraces are not. They were probably shaped during the neolithic era, transforming a striking natural feature into a supernatural monument. The tower on the summit is the surviving part of the church of St Michael. Established in the 12th century, the church benefited from pilgrims flocking to Glastonbury in the belief it was the holy Isle of Avalon – last resting place of both the holy grail and King Arthur. This was the height of the church’s power, and pilgrims’ willingness to believe. During Henry VIII’s dissolution of the monasteries in the 1530s, fortunes on the tor changed. The king disbanded the powerful religious houses and seized their assets, and in 1539 the elderly abbot was hanged, drawn and quartered on the walls of St Michael’s as a punishment for concealing religious treasures from the inventory. The church was destroyed and the stone carted away.

• Secret Britain by Mary-Ann Ochota is published by Frances Lincoln on 29 September, £20. Mary-Ann will be in conversation with Prof Francis Pryor about Secret Britain on Tuesday 6 October, 7pm, live online. Tickets £6, with a signed book, £20