US history doesn't start with the Declaration of Independence in 1776 but millions of years before that, with a host of discovered artefacts – from ancient rock carvings to remnants of a lost colony – leaving us with many more questions than answers. Who made them? Why? And what were they used for? Listed from oldest to newest (plus dates we still don't know), these archaeological discoveries in the US remain unexplained...

Evidence that humans roamed North America between 21,000 and 23,000 years ago was revealed with the discovery of ancient, fossilised footprints in the White Sands National Park in New Mexico. These dates were suggested in September 2021, and by November 2022 a new study had already disputed them. It argued that the footprints could have been left thousands of years later and are actually between 15,000 and 13,000 years old. The prints were found buried in layers of sediment across the ancient Lake Otero and belonged mostly to children and teenagers. 

Poverty Point's ginormous earthen mounds and concentric half-circles were shaped by hand, without domestic animals and centuries before the wheel was invented. The oldest, Lower Jackson Mound, has been dated to around 3500 BC. Ongoing archaeological studies suggest it was an ancient residential, trade and ceremonial centre, but we still aren't 100% sure. It's thought that Poverty Point was abandoned sometime around 1100 BC and that another Indigenous group moved in around AD 700 – but only for a brief period.

OK, so it's not as mighty as England’s famous stone circle, but America’s Stonehenge is also shrouded in mystery. Nobody really knows who constructed the man-made chambers, walls and ceremonial meeting places, but parts of the site have been carbon-dated to 4,000 years ago. There are supposedly Phoenician, Iberian Punic and Ogham (an early Celtic alphabet) inscriptions, although some think these could have been left by later Indigenous peoples or be colonial graffiti.

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The fascinating Great Serpent Mound is more than 1,300 feet (396m) long and 20-25 feet (6-7.6m) wide, and protrudes from the grassy ridge by up to 5 feet (1.5m), making it the world's largest serpent effigy. It winds across a plateau in a snake-like shape and is believed to have been constructed by Indigenous Ohioans. Archaeologists haven’t been able to specify the culture or date: estimates wildly vary from 321 BC to AD 1070. There are also three burial mounds nearby – two created by the Adena culture between 800 BC and AD 100 and one by the Fort Ancient culture (AD 1000-1650).

For hundreds or maybe thousands of years, the Hagood Creek Petroglyphs lay hidden beneath a dirt road before being excavated in 2003. The boulder's 32 distinct petroglyphs show 18 humans and several abstract images. Today the ancient rock carvings are on display at the Hagood Creek Petroglyph Site. But experts haven't been able to pinpoint a specific era, nor exactly what the carvings were trying to communicate.

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During 19th-century excavations of Grave Creek Mound (built between 250 and 150 BC), archaeologists uncovered this little sandstone disk dubbed the Grave Creek Stone. Some think it could be a hoax item, but attempted translations suggest Iberian Punic script, Old European or Libyan (depending on which way you turn the stone). Pictured here is the original beside one of its many copies.

The Great Gallery is the best known pictograph panel in Horseshoe Canyon, with well-preserved, life-sized figures (minus limbs) forming some of the most significant rock markings in North America. While they're characteristic of the Barrier Canyon style (2000 BC-AD 500), their exact dates are hard to know. Initially they were thought to be 2,000-4,000 years old, but in 2016 another study suggested instead that they were 900-2,000 years old.

When you imagine Miami, you likely think of candy-coloured lifeguard huts, not pre-Columbian stone circles. And you'd especially not expect to find them slap-bang in the middle of downtown. The site was first discovered in 1998 and consists of 24 large holes, which form a circle nearly 40 feet (12m) in diameter. Within this are hundreds of smaller, rectangular holes. They may have served as post-holes for the wall of a circular structure, but there’s no solid evidence.

This curious petroglyph is made of interconnected shapes that form a pattern of mazes. The symbol has been used in Indigenous American and Asian art for millennia, yet we don’t know exactly who made this drawing or how old it is. Chinese Buddhist monks? Shipwrecked sailors? The prevailing theory attributes it to Indigenous Californian artists, as it’s similar to the Rancho Bernardo style found in other rock art in southern California.

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The Blythe Intaglios are like an American version of Peru’s Nazca Lines. The oversized figures – the largest is 171 feet (52m) long – were probably created between 450 and 2,000 years ago and are generally credited to the Mohave and the Quechan. We don't know their purpose, but they may be connected to sacred ceremonial dances once held in the area. They were only discovered in the 1930s by a pilot flying overhead.

In 1680, John Danforth spotted a partially submerged rock with mysterious carvings in the Taunton River. Dighton Rock is now preserved at a small museum, but we still don’t know for sure who made the carvings – or why. It’s most likely the work of Indigenous Americans depicting an encounter or battle with visitors arriving by ship. But other theories claim it could be of Phoenician, Viking, Japanese, Chinese or even alien origin.

Could there really be a hidden city in Death Valley National Park? According to a 1947 report, in 1931 a man discovered a "series of complex tunnels deep below Death Valley". One tunnel supposedly led to the surprising discovery of three mummified 'giants' eight to nine feet (just under three metres) tall, hieroglyphics and an alleged ritual hall. There are other accounts of people discovering this mysterious spot too, and it matches a Paiute legend about the Kingdom of Shin-au-av and a journey to an underground cavern…

Largely believed to be Cherokee artwork, the Judaculla Rock is a sandstone boulder with some 1,500 designs thought to have been carved between 1,500 and 300 years ago. According to Cherokee legend, it could have been a boundary marker for hunting grounds which the giant Judaculla, master of animals, guarded. But some scientists argue that the Cherokee weren’t known for this type of artwork; instead, it could be a remnant of an Ice Age tribe. And then there’s the striking similarities with prehistoric stones in Scotland, nearly 4,000 miles (6,437km) across the Atlantic...

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Spread across 2,200 acres with a peak population of 25,000-50,000, Cahokia was built around AD 1050 with over a hundred earthen mounds and a series of ‘Woodhenges’ – calendars made with red cedar posts that lined up with the rising sun on certain days of the year. Experts still aren’t sure why they chose to build on a flood-prone area, nor why the site was eventually abandoned in AD 1350.

Built by the ancestral Sonoran Desert people, Casa Grande was once part of a larger complex of adobe structures which contained a ball court and was surrounded by a wall. The site was abandoned around AD 1450 – some 50 years before Columbus arrived – and rediscovered in the 17th century, when it was recorded simply as ‘Casa Grande’. Experts can’t work out what purpose the four-storey building (pictured) served: a local gathering point, perhaps, or a waypoint marker?

Bighorn Medicine Wheel, in northern Wyoming’s Bighorn National Forest, is a stone circle usually covered in snow during winter. Come spring the snow melts away to reveal limestone rocks scattered in a wheel shape with spokes encased in a large circle. The mountaintop site dates to at least AD 1200 and forms part of a chain of Native American archaeological sites up to 7,000 years old. It’s thought that the pattern was used to predict astronomical events such as the summer solstice, though no one knows for sure.

Constructed between AD 1250 and 1350 by pre-Columbian Mogollon people, Kinishba was a large Pueblo village that was abandoned in the late-14th century for reasons unknown. Nine buildings up to three stories high once contained 400-500 rooms, which housed some 1,000 occupants. The site lies in a grassy valley on land belonging to the White Mountain Apache tribe, so visitors are required to check in at the Nohwike’ Bagowa Museum before visiting.

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Rediscovered in Minnesota in 1898, the Kensington Runestone is inscribed with Scandinavian letters recording the movements of Vikings as they travelled "on an exploration journey from Vinland to the west". It gives a date of AD 1362 (some 130 years before Columbus's first voyage). Later examinations seemed to confirm that it was actually a 19th-century production – but some are still adamant (or hopeful) that it’s from the 14th century.

Nobody knows who built this series of stone structures, also known as the East Bay Walls. They sprawl some 50 miles (80km) through the hills between East Bay in Berkeley and San Jose, but are too low to be a defensive barrier. Among other oddities, one section precisely encircles a large boulder. Spanish settlers reported they were already here when they arrived, and when they asked the local Indigenous Ohlone peoples, they said the same thing. All in all, 'Berkeley Mystery Walls' is quite a fitting name.

Fort Mountain State Park is named after this 885-foot-long (269m) wall of loose rocks. A plaque marker at the site explains it could have been a prehistoric structure to do with sun worship or defence, the product of warfare between rival Indigenous American tribes, a defence fortification for gold-hunting Spanish conquistadors or a honeymoon haven for newly-married Cherokees. Others believe it was built by the 'moon-eyed people' of Cherokee folklore. 

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There are more than 300 stone chambers dotted around New England, with multiple theories claiming they’re of Indigenous American or early European settler origin. Pictured here is Nashoba Brook Stone Chamber in Acton, Massachusetts; a nearby sign states that it was most likely built in the 18th century, but masonry evidence hints that it was constructed before colonial times. Some think these stone chambers could once have been used as spiritual sites, or by early farmers.

In 1587, English settlers established a colony on Roanoke Island in North Carolina. But by 1590, all traces of the settlement had mysteriously disappeared. The only clue was the word 'Croatoan' carved into a post, and the fates of the 115 men, women and children became one of America's biggest mysteries. Experts think they might have cracked the case following the discovery of English pottery at another site. They believe the discovered relics belonged to the vanished colonists, though there are still skeptics. 

In October 2022, this rusty old boat was discovered in drought-ridden Lake Shasta. It’s the remains of a Higgins ship which was used during the Second World War for D-Day invasions in the Pacific, where it was previously thought to have sunk. At the moment, no one is quite sure how it ended up at the bottom of Lake Shasta, with officials dubbing it ‘the ghost boat’.

When you hear the word ‘Roswell’ it’s hard not to think of aliens, or at least UFOs. In 1947 this relatively small New Mexico city made headlines when a ranch worker claimed to have discovered the debris of a flying saucer. Army officials explained it was a weather balloon crash but many didn’t believe the story – and still don’t. Today Roswell is at the heart of countless alien conspiracy theories, and embraces its role with dozens of gift shops, museums and an annual UFO festival.

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