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Scientists unravel the mystery of the Roman 'gate to hell'

By Nell Lewis, for CNN

Updated 2119 GMT (0519 HKT) March 9, 2018

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Scientists believe they have figured out the mystery behind a Greco-Roman temple where animals were reported to drop dead as part of a ritual sacrifice. Known as the "Plutonium," the temple consisted of a stone doorway that led to a cave-like grotto, established around 190 BC. A team of Italian archaeologists rediscovered the cave in 2013 -- this is their 3D virtual reconstruction of the site.
Photos: The science behind the Roman 'gate to hell'
Scientists believe they have figured out the mystery behind a Greco-Roman temple where animals were reported to drop dead as part of a ritual sacrifice. Known as the "Plutonium," the temple consisted of a stone doorway that led to a cave-like grotto, established around 190 BC. A team of Italian archaeologists rediscovered the cave in 2013 -- this is their 3D virtual reconstruction of the site.
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These are the present-day remains of the Plutonium, in Hierapolis. In ancient times it was believed to be the gateway to the underworld as animals, led into the cave by Roman priests, would die of asphyxiation. The phenomenom continues today, as small birds and beetles have been found dead at the same site.
Photos: The science behind the Roman 'gate to hell'
These are the present-day remains of the Plutonium, in Hierapolis. In ancient times it was believed to be the gateway to the underworld as animals, led into the cave by Roman priests, would die of asphyxiation. The phenomenom continues today, as small birds and beetles have been found dead at the same site.
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During excavation, archaeologists found evidence of benches surrounding the Plutonium -- as this 3D virtual reconstruction shows. Visitors could watch the animal sacrifices, at a safe distance from the cave. New research shows that a fissure in the earth's surface, deep beneath the cave, emits carbon dioxide at such high levels that it can be deadly.
Photos: The science behind the Roman 'gate to hell'
During excavation, archaeologists found evidence of benches surrounding the Plutonium -- as this 3D virtual reconstruction shows. Visitors could watch the animal sacrifices, at a safe distance from the cave. New research shows that a fissure in the earth's surface, deep beneath the cave, emits carbon dioxide at such high levels that it can be deadly.
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An inscription discovered by archaeologists shows that the Ancient Romans dedicated the site to Pluto, god of the underworld and his wife Kore (Persephone). As the<a href="https://www.britannica.com/topic/Persephone-Greek-goddess" target="_blank"> myth</a> goes, Kore was abducted by Pluto and taken to the underworld. Jupiter, king of the gods, ordered her release, but because she had eaten a single pomegranate seed in the underworld she could not be completely freed, and spent one-third of each year with Pluto.
Photos: The science behind the Roman 'gate to hell'
An inscription discovered by archaeologists shows that the Ancient Romans dedicated the site to Pluto, god of the underworld and his wife Kore (Persephone). As the myth goes, Kore was abducted by Pluto and taken to the underworld. Jupiter, king of the gods, ordered her release, but because she had eaten a single pomegranate seed in the underworld she could not be completely freed, and spent one-third of each year with Pluto.
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The Plutonium was situated beside the Temple of Apollo, in the ancient city of Hierapolis (modern-day Pamukkale in southwestern Turkey).
Photos: The science behind the Roman 'gate to hell'
The Plutonium was situated beside the Temple of Apollo, in the ancient city of Hierapolis (modern-day Pamukkale in southwestern Turkey).
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The ruins of Hierapolis -- now a UNESCO World Heritage Site -- attract thousands of tourists every year.
Photos: The science behind the Roman 'gate to hell'
The ruins of Hierapolis -- now a UNESCO World Heritage Site -- attract thousands of tourists every year.
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The site includes the remains of a <a href="http://whc.unesco.org/en/list/485" target="_blank">Roman amphitheater</a>, built in the 2nd century AD under Emperor Hadrian. It was later renovated under Septimius Severus.
Photos: The science behind the Roman 'gate to hell'
The site includes the remains of a Roman amphitheater, built in the 2nd century AD under Emperor Hadrian. It was later renovated under Septimius Severus.
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Hierapolis is also famous for its hot springs and <a href="http://whc.unesco.org/en/list/485" target="_blank">travertine terraces</a>, formed by deposits of calcium carbonate. At the end of the 2nd century BC, the dynasty of the Attalids, the kings of Pergamon, established the city as a thermal spa.
Photos: The science behind the Roman 'gate to hell'
Hierapolis is also famous for its hot springs and travertine terraces, formed by deposits of calcium carbonate. At the end of the 2nd century BC, the dynasty of the Attalids, the kings of Pergamon, established the city as a thermal spa.
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This use continues today, as the calcium-rich water is reputed to have healing qualities for eye and skin diseases.
Photos: The science behind the Roman 'gate to hell'
This use continues today, as the calcium-rich water is reputed to have healing qualities for eye and skin diseases.
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The Plutonium is not the only example of an ancient site centered around noxious fumes. Ancient literature suggests that at the Palace of Apollo in Delphi, Greece -- where the oracle once prophesied the future -- people would enter a trance after inhaling noxious fumes. Modern research puts this down to a mixture of carbon dioxide and methane coming from an earth fissure.
Photos: The science behind the Roman 'gate to hell'
The Plutonium is not the only example of an ancient site centered around noxious fumes. Ancient literature suggests that at the Palace of Apollo in Delphi, Greece -- where the oracle once prophesied the future -- people would enter a trance after inhaling noxious fumes. Modern research puts this down to a mixture of carbon dioxide and methane coming from an earth fissure.
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Plutonium 3D close upPlutonium entrancePlutonium 3D from afarPlutonium inscriptionPlutonium temple of apolloPlutonium HierapolisPlutonium Hierapolis theatrePlutonium travertines Plutonium hierapolis travertine poolsPlutonium delphi

Story highlights

  • A grotto in Turkey was once believed to be the gateway to hell
  • Animals mysteriously died at its entrance
  • Two thousand years later scientific research reveals the reason

(CNN)Two thousand years ago, ancient tourists flocked to a Greco-Roman temple in Hierapolis, modern-day Turkey, situated on top of a cave once purported to be the gateway to the underworld.

They watched in awe as animals, from birds to bulls, dropped dead at its entrance. The cave, named the "Plutonium" after Pluto, god of the underworld, was thought to belch the "breath of death," killing all those in its reach, except the divinely immune priests who led the animals to sacrifice.
    Roman author and natural historian Pliny the Elder described the phenomenon as the "sewer of Charon" -- the mythical ferryman who rowed souls across the River Styx and Acheron and into the depths of the underworld.

    Toxic gas

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    But scientists have now provided an explanation for the mystery, and it's not supernatural. Research published by the journal of Archaeological and Anthropological Sciences in February shows that a fissure in the earth's surface, deep beneath the site, emits carbon dioxide at concentrations so high it can be deadly.
    Using a portable gas analyzer, Hardy Pfanz and his team of volcanologists found CO2 at levels ranging from 4-53% at the mouth of the cave, and as high as 91% inside -- more than enough to kill living organisms.
    Entrance to the Plutonium
    Entrance to the Plutonium
    "Problems for mammals (including humans) start way below 5% CO2," Pfanz told CNN. "A longer stay at 7% and more leads to sweating, dizziness, tachycardia (elevated heart rate) etc. A further increase would lead to asphyxiation due to the lack of oxygen and due to acidification of the blood and the body or brain cells."
    So it's no wonder the animals that entered the cave came to a swift end. During the research period alone, Pfanz says they found several dead birds, mice and more than 70 dead beetles.
    From the Romans to Byzantines to Ottomans and everyone in between, the Anatolian peninsula contains the legacies of a number of empires. The Library of Celsus in the ancient Greek city of Ephesus in modern day Izmir. Once a key locale for Greece on Asia Minor, the city in western Turkey has origins dating back to the 7th century BC.
    Photos: Anatolia's archeological treasures
    The Library of Celsus, Ephesus – From the Romans to Byzantines to Ottomans and everyone in between, the Anatolian peninsula contains the legacies of a number of empires. The Library of Celsus in the ancient Greek city of Ephesus in modern day Izmir. Once a key locale for Greece on Asia Minor, the city in western Turkey has origins dating back to the 7th century BC.
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    Said to have capacity for 25,000 people, its size helps archaeologists understand the scale of the ancient city's population. Dating from the 3rd century BC, the Hellenistic structure played a part in entertainment as well as political and religious gatherings.
    Photos: Anatolia's archeological treasures
    The Great Theater, Ephesus – Said to have capacity for 25,000 people, its size helps archaeologists understand the scale of the ancient city's population. Dating from the 3rd century BC, the Hellenistic structure played a part in entertainment as well as political and religious gatherings.
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    The final resting place of "Amyntas, son of Hermagios" dates from the mid-4th century BC. Cut into the hillside overlooking the modern city of Fethiye, close to the Aegean Sea, it was built by the Lycians of Telmessos, a city-state that would go on to be conquered by Alexander the Great.
    Photos: Anatolia's archeological treasures
    The Tomb of Amyntas, Telmessos – The final resting place of "Amyntas, son of Hermagios" dates from the mid-4th century BC. Cut into the hillside overlooking the modern city of Fethiye, close to the Aegean Sea, it was built by the Lycians of Telmessos, a city-state that would go on to be conquered by Alexander the Great.
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    Three miles south of Belkis in southern Turkey are the ruins of Aspendos, a Greco-Roman city that passed into the hands of the latter in 189 BC. The site's colossal theater dedicated to Marcus Aurelius remains its star attraction.
    Photos: Anatolia's archeological treasures
    Roman theater, Aspendos – Three miles south of Belkis in southern Turkey are the ruins of Aspendos, a Greco-Roman city that passed into the hands of the latter in 189 BC. The site's colossal theater dedicated to Marcus Aurelius remains its star attraction.
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    A full solar eclipse in Side, 2006. Founded by Aeolian Greeks, invaded by Alexander and at one time turned into a slave market by Cilician pirates, perhaps its most famous landmark is the Hellenic Temple of Apollo (pictured).
    Photos: Anatolia's archeological treasures
    Temple of Apollo, Side – A full solar eclipse in Side, 2006. Founded by Aeolian Greeks, invaded by Alexander and at one time turned into a slave market by Cilician pirates, perhaps its most famous landmark is the Hellenic Temple of Apollo (pictured).
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    Locals watch the tomb of Zenyel Bey transported in May 2017. The 15th century monument, dedicated to a figure from the Ak Koyunlu, a Turkmen tribe, was moved away from ground at risk of flooding due to a hydroelectric dam project in southeast Turkey. The Ak Koyunlu, which translates as "White Sheep," once ruled Anatolia, Azerbaijan and northern Iraq in the 14th to early-16th century AD.
    Photos: Anatolia's archeological treasures
    The tomb of Zenyel Bey, Hasankeyf – Locals watch the tomb of Zenyel Bey transported in May 2017. The 15th century monument, dedicated to a figure from the Ak Koyunlu, a Turkmen tribe, was moved away from ground at risk of flooding due to a hydroelectric dam project in southeast Turkey. The Ak Koyunlu, which translates as "White Sheep," once ruled Anatolia, Azerbaijan and northern Iraq in the 14th to early-16th century AD.
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    Other monuments from the ancient city on the banks of the Tigris will also be moved according to CNN Turk, including tombs, mosques and minarets. Hasankeyf has a 12,000-year history and contains many neolithic caves.
    Photos: Anatolia's archeological treasures
    Hasankeyf – Other monuments from the ancient city on the banks of the Tigris will also be moved according to CNN Turk, including tombs, mosques and minarets. Hasankeyf has a 12,000-year history and contains many neolithic caves.
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    Xanthos was the principal city of Lycia, in southwest Turkey. Mentioned in Homer's "Iliad," set during the Trojan Wars, the city was later attacked in the 6th century BC by Persian king Cyrus II.
    Photos: Anatolia's archeological treasures
    Xanthos – Xanthos was the principal city of Lycia, in southwest Turkey. Mentioned in Homer's "Iliad," set during the Trojan Wars, the city was later attacked in the 6th century BC by Persian king Cyrus II.
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    The city rose again and fought the Romans in the 1st century BC. Xanthos' rock-cut tombs are among its best known features, like this Lycian sarcophagus dated from the 4th century BC.
    Photos: Anatolia's archeological treasures
    The city rose again and fought the Romans in the 1st century BC. Xanthos' rock-cut tombs are among its best known features, like this Lycian sarcophagus dated from the 4th century BC.
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    Allianoi in eastern Turkey was once a significant Roman thermal spa famed for its spring waters and healing center. Pictured in 2008, the archaeological site near Bergama dates back to the second century AD. The site was discovered in 1998 during routine excavation on a proposed dam site and was flooded after the construction of the Yortanli Dam.
    Photos: Anatolia's archeological treasures
    Roman spa, Allianoi – Allianoi in eastern Turkey was once a significant Roman thermal spa famed for its spring waters and healing center. Pictured in 2008, the archaeological site near Bergama dates back to the second century AD. The site was discovered in 1998 during routine excavation on a proposed dam site and was flooded after the construction of the Yortanli Dam.
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    The dam is the same to later threaten Hasankeyf. Some items including statues were removed from Allianoi before workers reburied the spa in sand in September 2010 to protect it from the coming flood waters.
    Photos: Anatolia's archeological treasures
    Roman spa, Allianoi – The dam is the same to later threaten Hasankeyf. Some items including statues were removed from Allianoi before workers reburied the spa in sand in September 2010 to protect it from the coming flood waters.
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    Infrastructure work in Istanbul has revealed layers of the city's history for over a decade. In 2006 Byzantine ships were discovered as part of the Marmaray tunnel project across the Bosphorus strait; relics of the ancient port of Theodosius, once thought to be lost by archaeologists. The tunnel opened in 2013, but not before digs had yielded a huge number of <a href="http://edition.cnn.com/2009/WORLD/meast/09/21/turkey.bosphorus.tunnel.marmaray/">ships, along with jetties and docks</a>.
    Photos: Anatolia's archeological treasures
    Byzantine skipwrecks, Istanbul – Infrastructure work in Istanbul has revealed layers of the city's history for over a decade. In 2006 Byzantine ships were discovered as part of the Marmaray tunnel project across the Bosphorus strait; relics of the ancient port of Theodosius, once thought to be lost by archaeologists. The tunnel opened in 2013, but not before digs had yielded a huge number of ships, along with jetties and docks.
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    ephesus izmir turkeyephesus izmir turkey 2tomb of amyntas turkey aspendos theater turkeygreek temple of apollosie turkeytomb of zenyel bey turkey hasankeyf turkeyrestricted xanthos ruinslykian sarcophagus xanthosallianoi thermal spring izmirallianoi thermal spring izmir 2istanbul byzantine ship

    Tourist attraction

    It was no different in ancient times, when people traveled to Hierapolis to see the phenomenon. The city, founded around 190 BC, was famous for the Plutonium as well as its hot springs, which still attract thousands of tourists today. Francesco D'Andria, the Italian archaeologist who rediscovered the site in 2013, says that they found evidence of a theater around the Plutonium that could have been for spectators.
    A reconstruction of the complex surrounding the Plutonium, including benches for specators
    A reconstruction of the complex surrounding the Plutonium, including benches for specators
    The Greek geographer Strabo, who lived from around 64 BC to 21 AD, described the spectacle: "Any animal that passes inside meets instant death. At any rate, bulls that are led into it fall and are dragged out dead; and I threw in sparrows and they immediately breathed their last and fell," he wrote.
    Strabo recognized this reaction was related to the emission of gas -- "the space is full of a vapor so misty and dense that one can scarcely see the ground" -- but he puzzled over how it affected the animals but not the priests, asking whether it was because of their divine providence or simply because they held their breath.
    The Plutonium was situated beside the Temple of Apollo
    The Plutonium was situated beside the Temple of Apollo
    Pfanz's research adds another possibility: the fact that the animals and priests are different heights. CO2 is a heavier than oxygen, therefore it settles lower, forming a toxic gas lake above the ground. "The nostrils of the animals were way in the gas lake," he says, whereas the priests stood taller, above the gas lake.
      Pfanz believes the priests were aware of the gas and knew that its concentration fluctuated depending on the time of day. Recent research, which took measurements over a period of time, found that the CO2 level was particularly high at dawn and dusk, as sunlight disperses the gas.
      But D'Andria disputes this, citing a number of oil lamps found in front of the grotto. "It was highly probable that many ritual activities were developed during the night," he says.