From Rome's Colosseum to Stonehenge in the UK, the world's ancient sites typically draw thousands of visitors each year keen to unlock the mysteries of past civilisations. But experts are unearthing secrets about these intriguing sites all the time, and it's clear we've only just scratched the surface of what there is to learn. Here we reveal some of the most fascinating discoveries from recent years.

Hierapolis is an ancient Turkish city thought to date back to the 2nd century BC. The site sits close to the travertine pools of Pamukkale and it draws tourists with its atmospheric ruins, from crumbing baths to temple buildings. But there's one Greco-Roman temple in particular that has a peculiar story to tell. The temple is called Plutonium, named after Pluto, the God of the Underworld, and has long been dubbed the 'gate to hell'.

In ancient times, animals would be led to Plutonium by priests as part of a sacrificial ritual, and the crowd would look on in awe as the beasts dropped dead once inside the temple's cave-like interior. This earned the building a reputation as a passage to the underworld. Today, birds and insects have been known to perish upon entering and scientists recently discovered the cause – a fissure deep beneath the structure that releases toxic levels of carbon dioxide.

It might not be the most famous of Egypt's pyramids, but this striking structure in Dahshur still has secrets aplenty. Reopened to the public in 2019 for the first time since the 1960s, the Bent Pyramid (so named for its curious shape) sits around 25 miles (40km) south of capital Cairo. It's thought to be around 4,600 years old, dating back to 2,600 BC, when it was built for the pharaoh Sneferu. But despite its many millennia, its riches are only just being discovered.

The 331-foot (101m) structure has benefited from a year of restoration efforts, and excavations all around the vast Dahshur pyramid site (home to the Pyramid of King Amenemhat II and the Black Pyramid too) have revealed a whole host of treasures. These included a network of hidden tombs, and a range of sarcophagi containing mummies dating to the Late Period (664–332 BC). Archaeologists also found stone-cutting tools and funerary masks.

Further south, in the Valley of the Kings, the last resting place of King Tutankhamun is still yielding secrets. And some are quite literally out of this world. When Howard Carter opened the tomb in 1925, two daggers were found with the sarcophagus, one of which was resting on the right thigh of the king. Studies have now confirmed this weapon is made from iron that probably came from a meteorite. The composition of the 13.4-inch (34.2cm) long dagger's blade contains a high amount of nickel that's not found anywhere on Earth, suggesting to scientists that its raw materials were extraterrestrial.

You've no doubt heard of Machu Picchu, Peru's famous Inca citadel, but Chan Chan, a mammoth pre-Columbian site further north, often flies under the radar. Once the thriving capital of the Chimú civilisation, Chan Chan had its heyday in the 15th century, and today it's a tangle of hulking walls and passageways endangered by the area's heavy rain. Nevertheless, research is ongoing and recent explorations of the site have proved extremely fruitful.

An intricate adobe wall and some 19 wooden statues were discovered on the site at the end of 2018. It's thought that the structures were buried around 750 years ago and each one stands around 26 inches (70cm) tall, cast in wood, with a simple clay mask. The figures also seem to wield both a sceptre and a shield. Experts believe the statues could have served as "guardians" and excavation work is set to continue.

One of Istanbul's most striking sights, Hagia Sophia draws tourists in their droves with its domes, minarets and mosaic-filled interiors. Dating to the 6th century, the structure began life as a Byzantine church, before becoming a mosque in the 1400s. Since 1934 it has served primarily as a museum but in July 2020 it was announced Hagia Sophia would return to being a Muslim place of worship. Its secrets are only just coming to light and research at this spectacular site is ongoing and innumerable treasures have turned up in recent years. 

One of the most exciting finds is the Great Baptistery, where it's thought that emperors baptised their kin for more than a millennia. Around the same time, researchers discovered a space they believe was a library, once home to around 1,000 ancient scrolls. Finally, alongside hordes of mosaics, graffiti and frescoes hiding beneath more modern plasterwork, experts uncovered a circular spot that may look like nothing to the untrained eye. However, those in the know think this was where Byzantine emperor Justinian I would have stood during religious ceremonies.

Often overshadowed by Rome's showier sights, the Domus Aurea (interior pictured) was once a glittering palace, the vision of Roman emperor Nero in AD 64. However, following Nero's death, his successors vowed to get rid of the flamboyant structure, destroying its upper levels and burying lower parts entirely. The Baths of Trajan were eventually built over the top of the palace, and it was as if it never existed.

Thankfully, though, the palace complex was preserved underground, and it's still revealing fresh secrets. The latest discovery is an entire room, happened on by chance during restoration work. Thought to have been constructed between AD 65 and 68, the Sphinx Room, as it has been named, is a small chamber adorned with murals, from garlands and fruit to centaurs and a lone sphinx. Though there's more of the space to be discovered, experts are reluctant to excavate further for fear of making the structure unstable.

Rome's colossal amphitheatre features high on many a bucket list and it's a place ripe for archaeological discoveries. Construction of the Colosseum began around AD 70 for Roman emperor Vespasian and, upon its opening, thousands of viewers would come to witness bloody gladiatorial games and other spectacles. But recent studies of the site have revealed more of the historic arena's hidden secrets. Firstly, during renovation efforts in the past few years, workers discovered a series of red numbers painted into the amphitheatre's arches. 

The exposed numbers indicate that Roman spectators at the Colosseum followed an ordered seating plan and system of entry, not too dissimilar to those we'd expect at modern concert venues and theatres today. Even more recently, in 2017, renovation efforts led to more surprises. This time discoveries were linked to a medieval baronial family (the Frangipane), who existed after the fall of the Roman Empire, and who built a fortress into the Colosseum's southern side. 

Examinations discovered holes in the monument's stone – these would have supported beams for a wooden walkway that the Frangipane used to look out for enemies. While little is left of the medieval family's influence, these tiny details help open up a lesser-known portion of the Colosseum's history.

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The ruins of Tikal, an ancient Maya civilisation, take up pride of place in northern Guatemala's Tikal National Park. It's thought that the site could have been settled as early as 600 BC, but the imposing pyramidal temples that wow modern visitors were constructed later (between AD 600–900). Now a UNESCO World Heritage Site, Tikal is still giving up its secrets. As recently as 2018, advanced LiDAR (Light Detection And Ranging) technology helped experts make some fascinating discoveries there. 

The laser technology, which uses light to map previously concealed sites, revealed that the famed vestiges of Tikal are a mere fraction of the Maya ruins that exist here. Experts discovered previously uncharted palaces and fortresses, as well as ancient highways and irrigation systems. This new information has led scientists to conclude that the 800-square-mile (2,072sqkm) area of the Petén region surveyed could have supported a population of around 10-15 million people. This is compared with five million people, as originally thought. 

The ancient archaeological site of Angkor, with the sprawling Angkor Wat temple complex at its heart, has held UNESCO World Heritage status since 1992. But the city has been beguiling architecture buffs long before the Nineties. Angkor flourished under the Khmer Empire from around the 9th to the 15th centuries AD, and archaeologists are still poring over its historic expanse. And, in 2016, some fascinating findings were revealed...

LiDAR technology has been used to scan the area around Angkor Wat and has revealed entire cities beneath the surrounding forestland. The findings, which included what's thought to be a complex water system and numerous gardens, have challenged expert's existing ideas about the scale of the civilisation that existed here and its way of life.

Few ancient cities are quite as captivating as Petra in southeast Jordan. The "Red Rose City" is famed for the Treasury, a 131-foot-high (40m) monument complete with columns and intricate carvings, whose exact function is still unknown to experts. Petra's exact date of origin remains unconfirmed, but historians surmise the city had its heyday from the 1st century BC up until a shattering earthquake in the 4th century AD. Now archaeologists are continuing to unearth its delights.

In 2016, satellites revealed a monumental structure in the ancient city. A huge, elevated platform, said to be the length of an Olympic swimming pool, was brought to light just 0.5 miles (0.8km) from Petra's centre. Archaeologists have not yet made any firm conclusions about its purpose, but it's thought it could have been used for public displays or ceremonies.

Tucked away in the Andes Mountains, the Inca ruins of Machu Picchu remain one of the world's most lauded historic sites. More than half a million people usually journey here each year, often following the fabled Inca Trail. But the ancient site, built around the mid-15th century and made up of temples, plazas and a royal palace, has suffered under the weight of the huge visitor numbers. New restrictions were imposed in 2017 as a result, but still fascination with this ancient wonder is far from waning.

 

Archaeologists' interest in this Inca site is alive and well too, and a new study has revealed that a natural disaster shook the site during its construction, around AD 1450. In 2018, the Geological, Mining and Metallurgical Institute of Peru found damage to some of Machu Picchu's major landmarks (including the Temple of the Sun), which they attribute to a brutal earthquake. According to the experts, this caused the Incas to change their approach to Machu Picchu's construction part-way through.

Scientists say that two distinct architectural approaches are evident at Machu Picchu – earlier parts are built with smaller stones, while later structures are made with hardier giant blocks. The research reveals the Inca's astute awareness of the natural dangers they were faced with, and their ingenious ways of adapting to them.

The vast archaeological site of Teotihuacán is less than an hour north of Mexico City. Little has been concluded about the ancient city's origins, but it's thought to have flourished between 100 BC and AD 650, more than a millennia before it was settled by the Aztecs. The Pyramid of the Sun is the largest structure on the site, at more than 216 feet (66m) above the ground. But it is the smaller (140 feet/43m) Pyramid of the Moon that has yielded the most interesting secrets in recent years. 

Using complex electric resistance technology, archaeologists were able to map the area beneath the Pyramid of the Moon. They found a secret tunnel running around 26 feet (8m) beneath the pyramid, as well as a kind of chamber. Experts believe Aztec peoples could have used the chamber for funereal rituals or ceremonies, and therefore the tunnel itself could have signified the route to the underworld. The next step will be to explore these subterranean wonders and hopefully shed more light on the customs of this ancient civilisation.

Agra's Taj Mahal needs little introduction. An elegant feat of Mughal architecture, it was built in the 17th century for emperor Shah Jahān in memory of his beloved late wife Mumtāz Maḥal. Today it's still considered a monument to love. But brand-new research has thrown up some more secrets. Italian researcher and physicist Amelia Carolina Sparavigna discovered that the site was actually built to seamlessly align with the summer and winter solstice.

Using satellite technology, Sparavigna spent time plotting the path of the sun over the monument and found that its movements perfectly align with the Taj Mahal's gardens. According to Sparavigna, if you visited the Taj Mahal before sunrise on the day of the Summer Solstice (usually 21 June), and you stayed right until sunset, you'd see the sun beat a perfect path from a pavilion in the site's northeast to another pavilion in the northwest. As the sun moves overhead it beautifully frames the Taj Mahal's mausoleum and minarets. 

As is common with Mughal architecture, the Taj Mahal's gardens are thought to signify the Garden of Eden, whose rivers famously flowed to the four corners of the world. The path of the sun serves to highlight this symbolism. Experts also suggest that, on a more practical level, knowledge of the sun's path would have helped architects plot the Taj Mahal site precisely and symmetrically.

The ancient Maya city of Chichén Itzá, in Mexico's Yucatán Peninsula, is thought to date back to the 6th century. The site has many a treasure, from an intricate nunnery to a sacred cenote, but El Castillo is the most glittering jewel in Chichén Itzá's crown. The pyramid, whose name means 'The Castle', towers to 79 feet (24m). Its design, including 365 steps, one for each day of the year, is a nod to the Maya calendar. And today, it's still slowly relinquishing its secrets.

In 2016, scientists used imaging technology to peer inside El Castillo and found something amazing. Experts already knew of a second, smaller pyramid hidden away within El Castillo's outer shell, but this technology uncovered a third. It's thought that this third pyramid measures about 32 feet (10m), and the discovery has helped scientists conclude that Chichén Itzá's heyday and its construction can be split into three key eras. The new pyramid is believed to have been built in the early 'Pure Maya' era from around AD 500 to 800, before the arrival of the Toltec peoples. Until now, little has been known of this ancient period of the site's history.

Widely touted as having the world's oldest temple, the site of Göbekli Tepe in Turkey has its origins between 9,600 and 8,200 BC, dramatically predating better-known wonders, such as the revered Pyramids of Giza. The plot is made up of limestone megaliths and is said to have never been inhabited. Instead, experts believe Göbekli Tepe had a ceremonial or funereal purpose, and ancient peoples would come here to worship. Since the site was originally written off as a graveyard in the 1960s, real excavation work only began in the early 1990s. This means there's still much to be uncovered. 

In 2017, experts from the University of Edinburgh made an exciting breakthrough. After studying the carvings on Göbekli Tepe's stone pillars, they uncovered evidence that a comet struck Earth in 11,000 BC. Markings on a pillar nicknamed the 'Vulture Stone' turned out to be astronomical symbols that related to the comet and the constellations over the site at the time. Scientists then used technology to recreate the constellations, and nail down the exact time of the comet. The comet had great significance for this ancient community since it was linked to the start of a mini ice-age, which would deeply impact its people for years to come. 

The Chilean territory of Easter Island is famed for its gargantuan stone statues, or moai. There are hundreds of these sombre stone heads and torsos dotted across the isle, and while the exact date of their origin is unconfirmed, estimates usually fall between AD 1,050 and 1,680. The real mystery, though, is why these monuments were erected in the first place. But after years of deliberation, some scientists think they have the answers. By carefully mapping the moai's positions, experts found a correlation between the statues' locations and the presence of fresh water.

Though the island has no springs or streams above ground, researchers found that, near the coast, fresh water would surface from underground via a process called groundwater discharge. It could also have been found in caves. Though they're still unsure why the structures are so elaborate (some are more than 30 feet/9m), scientists are convinced they were in part built to signify and lay claim to this vital resource.

Most will have heard of Italy's Pompeii, the legendary Roman city that was destroyed by Mount Vesuvius' cataclysmic eruption in AD 79. Today what's left of Pompeii, including its ancient city gates, forum and basilica, draws tourists and historians from the world over. Research into this devastated city is ongoing, and conservation work in 2018 unveiled a previously unknown pocket of the site: the 'House of Jupiter'.

The buried 'House of Jupiter' (pictured) is an art-filled home thought to have belonged to a member of Pompeii's elite. Archaeologists believe it had been unearthed before, in the 18th or 19th centuries, but was forgotten as other parts of the site were excavated. Experts rediscovering the house found paintings (including one of god Jupiter, hence its nickname), as well as colourful wall designs and frescoes, coins and ornaments. This find offers revealing insights into the lifestyle of Pompeii's upper classes. Find more of Pompeii's secrets here.

China’s famous Great Wall is a series of ramparts and fortifications, spooling out for thousands of miles across the country. While the best-known portions of the wall have their origins in the Ming Dynasty (1368–1644), parts of it are thought to date back to 220 BC. Still the well-preserved Badaling section, around one hour 30 minutes from Beijing, remains the wall’s most touristed segment. But some of the more remote sections are suffering, crumbling and weather-beaten, and if they’re not restored soon they may fall into irreparable ruin.

Today, though, advanced drone technology is giving archaeologists the chance to explore previously inaccessible parts of the structure. It's allowing experts to acquire detailed site data, including the precise locations of battlements and arrow holes, and then produce 3D images. One affected section is Jiankou, a deteriorating chunk of the wall some 48 miles (77km) from Beijing. Archaeologists can then use the information gathered to plan restoration efforts and preserve this wonder for generations to come.

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This mighty Roman wall extends for some 73 miles (118km), reaching from coast to coast in northern England, and dating back to around AD 120. It was the vision of its namesake, emperor Hadrian, who wanted to clearly delineate Roman territory. Today, many visitors follow all or part of the designated Hadrian's Wall Path. Continued excavations of the surrounding areas have also offered further glimpses into Roman history, and experts hit the jackpot with one dig in 2017.

A Roman barracks at the fort of Vindolanda, just south of Hadrian's wall, was discovered in 2017, and excavated to reveal a whole heap of treasures. Findings included army accommodations, stables, fireplaces and pottery, as well as two wonderfully preserved Roman swords, an extremely rare find. Other discoveries included writing tablets, leather shoes, lances and arrowheads, and the value of the property left behind indicates that its occupants left in a hurry. Thought to have been established in AD 105, the barracks predate the wall. Thus, experts say, they offer an insight into the tumultuous stretch of time just before the wall's eventual construction.

These incredibly intricate rock-hewn temples and monasteries are close to the Indian village of Ellora in Maharashtra state. There are 34 in total, a mixture of Buddhist, Hindu and Jain monuments, and elaborate carvings of deities, flowers and life-size elephants fill the space. The caves are thought to date back to between AD 600 and 1,000, and until recently it has been a mystery as to how the monuments have remained so beautifully preserved.

According to new research, the caves have remained so wonderfully intact for more than 1,000 years because hemp, or cannabis, was historically used to conserve them. A pair of Indian researchers who studied samples of plaster from the structure, concluded that hemp was used to strengthen the buildings and to ward off harmful insects that would have damaged the carvings and cave paintings here. The discovery is fascinating since it shows the ingenuity of architects potentially working as early as the 6th century. 

One of the world's most mesmerising archaeological feats, the Great Pyramid is the largest of Egypt's three Pyramids of Giza, and one of the 'Seven Wonders of the Ancient World'. It originally soared to a mammoth 481 feet (147m), with construction beginning from around 2,550 BC. Today, the triad of pyramids attract millions of tourists each year. But despite their celebrity status, they still hold some secrets, one of which was revealed in 2018. 

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It has long been a mystery as to how the towering Great Pyramid was constructed more than four millennia ago. But a group of archaeologists from Cairo's French Institute for Oriental Archaeology and the University of Liverpool think they've cracked it. In an ancient quarry in Hatnub, where it's thought the pyramids' building blocks were made, the team discovered remains of a curious contraption, dating back to the era when the structures were erected. 

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The instrument, the team discovered, included a central ramp with a staircase on either side and several postholes. Experts believe this ramped device formed part of a system used to haul heavy rocks via a sled from the quarry, and transport them to the Great Pyramid's construction site. While not conclusive, this imaginative tool offers some insight into how this ancient wonder came to be.

 

Experts have mused over the significance and purpose of these prehistoric pillars in England for years. The most common consensus is that Stonehenge, on Wiltshire's Salisbury Plain, was used for religious or funereal ceremonies. The stone circle is thought to have been erected in around 2,500 BC and visitors have come from around the globe to gaze up at the imposing structures, and especially to see them silhouetted against a blazing sunset. While Stonehenge still holds many mysteries, experts believe they've unravelled a few of them. 

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In recent studies, scientists have mapped the underground area around Stonehenge, and made some remarkable discoveries. Results suggested that, historically, Stonehenge did not stand alone. In fact, after using new technology to map the area, archaeologists found evidence to suggest it was once part of a wider network of 17 structures. Meanwhile, an even more recent study has also shed some light on the Neolithic peoples who are thought to have built the monument.

The new research involved carrying out detailed tests on the previously excavated remains of 25 people buried in pits beneath the henge. It found that 10 of these people hailed from parts of southwest Wales, where some of Stonehenge's rock is also thought to come from. The other 15 people are believed to originate from somewhere in western Britain. 

Most recently, in July 2020, a study pinpointed where Stonehenge's large upright sandstone boulders, called sarsens, originated from. New chemical analysis of a stone that was removed during restoration works in 1958 shows it’s likely these huge blocks were quarried just 15 miles (24km) away in West Woods, Wiltshire. Further analysis shows that 50 of the 52 stones at the site have the same chemical structure, but where the other two boulders came from remains a mystery. Here, the stunning spot is pictured as the Comet NEOWISE passes over on 21 July 2020.

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