ANCIENT HIERAPOLIS
The Gate to Hell and the City of the Dead at Hierapolis
Ancient writers described a cave that killed instantly, and priests who walked in unharmed. The science behind Hierapolis's 'gate to hell' and its necropolis.
Check dates and availability ↗Strabo's sparrows: what the ancient sources actually describe
The Plutonium — or Ploutonion, sanctuary of the god Pluto — is one of antiquity's best-documented 'haunted' sites, because multiple Greek and Roman writers described it firsthand. The geographer Strabo, writing in the first century BC, recorded a fenced opening in the rock at Hierapolis that released a vapour lethal to any living creature that entered it, and claimed to have tested this himself by throwing sparrows toward the opening and watching them drop dead almost instantly. The historian Cassius Dio later noted that an amphitheatre-like viewing structure had been built around the site so pilgrims and visitors could watch from a safe distance. These aren't later legends grafted onto the ruins — they are near-contemporary eyewitness accounts of a phenomenon that turns out to have a real, physical cause.
The priests who could walk in and live
What puzzled ancient observers most was that certain priests — the Galli, eunuch devotees of the goddess Cybele, whose cult was closely tied to the site — could enter the fenced enclosure and emerge unharmed, apparently immune to a vapour that killed animals within seconds. Strabo speculated their survival came from castration itself; modern reconstructions favour a more mundane explanation, that the deadly gas settled in a low layer near the ground, and that priests who were reasonably tall, held their breath, or stood on raised stones could keep their heads above the worst of the concentration long enough to perform a ritual and step back out. Whether through genuine technique or theatrical showmanship for an audience, the effect cemented the site's reputation as a literal gateway to the underworld.
The modern science behind the ancient horror
Archaeologists and geologists who re-excavated the Plutonium in the 2010s confirmed the mechanism behind the legend: the cave still emits carbon dioxide from deep geological activity along the same fault system that feeds Pamukkale's thermal springs, and because CO2 is denser than air, it settles into a shallow, invisible 'lake' rather than dispersing. Measurements at the site found concentrations lethal to small animals and birds pooling within around 40 centimetres of the ground — enough to kill anything that puts its head into that layer, while leaving a standing adult's head in clean air above it. Modern access to the excavated chamber is tightly restricted for safety, so today's visitors view the site and its surrounding ruins rather than approaching the vent itself.
One of Anatolia's largest ancient cemeteries
A short walk from the Plutonium and the theatre, the Hierapolis necropolis spreads for roughly two kilometres along the roads leading out of the ancient city, and it is genuinely vast — one of the largest and best-preserved burial grounds to survive from the Greco-Roman world, encompassing well over a thousand tombs. Because Hierapolis was famous as a healing and thermal-spa destination, it drew a steady stream of seriously ill visitors hoping the waters would cure them; many who came did not recover, and the necropolis's scale reflects centuries of arrivals from across the Mediterranean who ended their journey there rather than at home. Tombs range from simple sarcophagi to elaborate house-shaped monuments and circular tumuli, many still bearing carved inscriptions naming the dead.
What to actually look for as you walk through it
The necropolis rewards slowing down rather than photographing and moving on. Look for the variety of tomb types side by side — round tumuli with stone retaining walls, freestanding sarcophagi with gabled lids, and small house-tombs built to resemble the homes of the living — reflecting centuries of use by families of different means and origins. Many sarcophagi carry Greek inscriptions recording names, professions and sometimes curses against anyone who disturbed the grave, a detail that brings the abstract idea of 'ancient burial customs' down to individual, named people. The necropolis sits along the main approach roads rather than being fenced off as a separate attraction, so most visitors walking between gates pass directly through it, whether or not they realise what they're looking at.
Why the underworld cult sat right beside the sun god's temple
It seems strange today that a shrine to Pluto and a temple to Apollo stood essentially side by side at Hierapolis, but the pairing reflects real ancient religious logic. Apollo, worshipped here partly in his oracular aspect, and the chthonic cult built around the gas vent were both connected to prophecy and contact with forces beyond ordinary human control — the ground itself was seen as a point of access to hidden knowledge, whether channelled through a sun god's oracle or a literal opening to the underworld. The Temple of Apollo's foundations still stand adjacent to the Plutonium's remains today, and walking between the two is a reminder that ancient visitors experienced Hierapolis as one coherent sacred landscape, not the separate 'sights' modern itineraries divide it into.