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.

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The discovery of fossilised footprints in White Sands National Park offered evidence that humans roamed North America 21,000 to 23,000 years ago, much earlier than previously thought. However, the findings of the original 2021 study were disputed by a second study in 2022 which claimed the footprints were just 15,000 to 13,000 years old.

Fast forward to 2023, when a third study reaffirmed the original findings. The confusion all came down to the dating techniques used, which are still up for debate in some scientific circles.

Poverty Point's huge earthen mounds and concentric half-circles were shaped by hand, without domestic animals, centuries before the invention of the wheel. The oldest, Lower Jackson Mound, has been dated to around 3500 BC.

Archaeological studies suggest it was an ancient residential, trade and ceremonial centre, but researchers still aren't 100% sure why this sophisticated earthwork complex was constructed. It's thought Poverty Point was abandoned sometime around 1100 BC and that another Indigenous group moved in for a brief period around AD 700.

It may not be as mighty as England’s famous stone circle, but America’s Stonehenge is equally shrouded in mystery – nobody really knows who constructed the manmade chambers, walls and ceremonial meeting places. Parts of the site have been carbon-dated to 4,000 years ago, while the stone chambers and walls are more reminiscent of early American settlers in the 1700s.

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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Great Serpent Mound is more than 1,300 feet (396m) long, 20 to 25 feet (6 to 7.6m) wide and protrudes from a grassy ridge by up to three feet (1m), 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.

However, archaeologists haven’t been able to specify the culture or date – or why it was constructed. Estimates 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 to 1650).

For hundreds or maybe thousands of years, the Hagood Creek Petroglyphs lay hidden beneath a dirt road, before being excavated in 2003. Thought to be 1,000 to 2,000 years old, the 32 distinct petroglyphs show 18 human and several abstract images.

Experts haven't been able to pinpoint a specific era or decipher exactly what the carvings were trying to communicate. They are on display at the Hagood Creek Petroglyph Site.

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During 19th-century excavations of Grave Creek Mound (a burial mound built between 250 and 150 BC), archaeologists uncovered a little sandstone disk dubbed the Grave Creek Stone. Some think it could be a hoax item, but translations have suggested Iberian Punic script, Old European or Libyan (depending on which way the stone is turned).

To add to the mystery, the current whereabouts of the stone are unknown. Pictured here is the original beside one of its many copies. 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 to AD 500), their exact dates remain unknown.

Initially they were thought to be 2,000 to 4,000 years old, but a 2016 study suggested that they were between 900 and 2,000 years old. The age of the significant rock art remains subject to debate.

When you imagine Miami, you likely think of beaches and nightlife, not pre-Columbian stone circles – especially not in the middle of downtown. First discovered in 1998, the ‘Miami Circle’ site 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. The site, thought to be 1,700 to 2,000 years old, can be found in a protected park by the Miami River.

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 it’s still not known exactly who made the abstract drawing or when.

Over the years, the petroglyph has been attributed to Chinese Buddhist monks and shipwrecked sailors. However, the prevailing theory suggests it was the work of 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.

The geoglyphs were only discovered in the 1930s by a pilot flying overhead. Their purpose is unknown. They may be connected to sacred ceremonial dances once held in the area or could even be ancient star maps.

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.

They are likely the work of Indigenous Americans depicting an encounter or battle with visitors arriving by ship. But other theories claim they could be of Phoenician, Viking, Japanese, Chinese or even alien origin.

Is there 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 (2.4 to 2.7m) tall, hieroglyphics and an alleged ritual hall.

There are other accounts of people discovering this mysterious spot. It matches a Paiute legend about the Kingdom of Shin-au-av and a journey to an underground cavern. However, it remains the stuff of folklore, conspiracy and speculation.

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 guarded by Judaculla, ‘Master of all Game Animals’.

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. There’s also a striking similarity 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 20,000 to 50,000, Cahokia was built around AD 1050. It consists of around 120 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 in a flood-prone area, nor why the site was eventually abandoned around AD 1350 to AD 1400.

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 or residence, perhaps, or as 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 1760 and forms part of a chain of Native American archaeological sites which are 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 1300 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 storeys 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 from the 19th-century – but some are still adamant that it’s from the 14th century. For over 100 years, scientists, geologists and linguists have been unable to come up with a definitive answer.

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 and the Indigenous Ohlone people both noted the structures were already in place when they arrived on the land – so 'Berkeley Mystery Walls' is quite a fitting name.

Fort Mountain State Park is named after this 885-foot (269m) wall of loose rocks. A plaque marker at the site suggests various theories as to its provenance. These include that it’s a prehistoric structure relating to defence or sun worship, the product of warfare between rival Indigenous American tribes, or a fortification for gold-hunting Spanish conquistadors.

It may even have been a honeymoon haven for newly married Cherokees or built by the 'moon-eyed people' of Cherokee folklore.

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There are anywhere between 300 and 800 stone chambers dotted around New England, with multiple theories claiming they’re of Indigenous American or early European settler origin. Some think these stone chambers could have been used as spiritual sites or as storage.

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. 

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 – it could refer to a Native American tribe or an island off the North Carolina coast.

The discovery of English pottery thought to belong to the colonists at a nearby site offered a clue as to the fates of the 115 men, women and children – but the truth remains elusive more than 430 years later.

In October 2022, the remains of a military Higgins boat were discovered in drought-stricken Lake Shasta. The 36-foot (11m) landing craft was used during World War II, including as part of the 1943 Battle of Tarawa in the Pacific, where it sank and was later salvaged.

Though the US army used Lake Shasta for training, how this particular boat got to the lake remains a mystery. The fact that the ‘ghost boat’ is only visible when the water levels in the lake drop, adds to its mystique.

When you hear the name ‘Roswell’ it’s hard not to think of aliens and UFOs. The link began in 1947, when a ranch worker in the New Mexico city claimed to have discovered the debris of a flying saucer.

Army officials explained the debris was from a high-altitude balloon crashing, but many didn’t believe the story – and still don’t. Roswell remains at the heart of countless alien conspiracy theories. The town embraces its alien connection, with gift shops, museums and an annual UFO festival.

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