The gate to hell hiding above the pools of Pamukkale

You climb the hillside for those famous white terraces, the frozen waterfall of travertine with warm turquoise water on every shelf, and it really is as lovely as the pictures. But at the top, in the ruins of the ancient city of Hierapolis, there is a small arched doorway that hardly anyone is told to look for.

The Romans called it the Ploutonion, the gate to hell, and they meant it. A deep crack in the rock beneath it leaks volcanic carbon dioxide, which seeps out as a faint mist and settles along the ground. The old geographer Strabo described priests leading bulls up to the opening while pilgrims watched, the priests strolled out completely unharmed. For centuries it looked like proof of the gods. It was really just physics. Carbon dioxide is heavier than air, so it forms an invisible lake barely knee high, reaching around ninety per cent right by the vent. A priest standing tall kept his head in clean air just above it. The vent was lost for a very long time and only rediscovered in 2013 and now a UNESCO World Heritage site.

I painted the terraces in watercolour and ink, all that soft white stone and pale blue water catching the light, with hot air balloons floating high over the frozen waterfalls and the whole time I kept thinking about the little doorway just out of frame, and the calm men who turned a bit of chemistry into a miracle. There's a poster with a letter home and a postcard if you'd like one.