The sea is bubbling off the pebbly southern shore of Kos, and a small but noisy crowd of people are bathing in a patch of shallow water marked out by stones. What's going on?
We didn't expect to find thermal springs at the end of our hike across the island. As we join the crowd and lower ourselves into the strangely warm seawater, laughing as hot currents mingle with cold, one of the bathers tells us that this coast is volcanic. Waters spill forth at temperatures of up to 60 degrees Celsius.
An idea is planted. Days later, we are boarding a morning ferry for the two-hour crossing to Nisyros, a tiny island that is a sleeping volcano. As part of the Dodecanese islands, Nisyros has been controlled by the Ottomans, the Italians and the Knights of St John, but its character is undeniably Greek. As our boat pulls into Mandraki, the main town (population 600), we check off the icons of Aegean island life: a domed church, waterfront cafes with wooden chairs and moustachioed patrons, blue and white flags, fishing boats and a whitewashed monastery on the hillside.
With no airport, Nisyros is not visited by package tourists. We call in at the first hotel we find on the promenade - it's a family-run place that includes a breakfast of coffee, fruit and yoghurt, and we get a room with a balcony overlooking the water.
On a little beach opposite, we watch a gaggle of white ducks go for a paddle in the sea. The island has little fresh water, so every resident must adapt. An hour later, the sun is stronger and the ducks are taking shade under a beach umbrella.
We've come here for the thermal baths. The hot springs have been used since ancient times. An inscription found in Roman ruins nearby says: 'Happy shall be the man who shall be cured in this way as soon as he passes by the olive gate of the baths of Hippocrates.'
The municipal baths were opened in the 19th century and occupy a building that looks like a Victorian hotel. At its reception, a cheerful woman gives us towels and directs us into separate tiled chambers where hot, sulphurous water flows into and out of individual baths. 'Twenty minutes,' she says, tapping her wrist, and is gone. Soon she is back, knocking on the door, and that is fine for me; I could only take 10 minutes in the steaming waters, but I feel invigorated. Back in the lobby, I gesture to pay, but there's no charge - the baths are free.
To cool down, we take a breezy table outside the baths and order lemonade. 'Vasilios! Vasilios!' the waitress calls out, banging a food bowl for the cafe's three-legged cat. But the patient animal has been waiting at her feet the whole time.
In Mandraki that evening, we eat grilled octopus at a restaurant in a square sheltered by fig trees. Afterwards we are given glasses of soumada, an almond-based drink made by the islanders.
On our second day, an ancient German bus - the only public transport on Nisyros - carries us up to the hilltop village of Nikia. From there, we get our first view of the caldera, the still-smoking crater of the volcano.
A stocky man in his fifties drags a suitcase out of the bus behind us, and half a dozen people cry out and run to meet him. He tells us he was born here and is visiting from Australia.
The timetable said the bus would call in later beside the crater. We follow a stone-slab path down from the village, picking our way between almond and olive trees. We see no people for the next three hours; goats are the only inhabitants.
We're wearing hiking boots, so when we reach the edge of the crater we decide to scramble down into it. We step into a yellow-stained, moonlike landscape where nothing grows. Bursts of pungent gas escape from the earth. The volcano has not erupted since 1887, but we don't hang around.
'Kalimera!' a woman calls out from her bicycle as we walk the promenade the next day - it's the lady from the municipal baths. We feel like we've lived here forever.
The nearest airport to Nisyros is on the neighbouring island of Kos from where there are regular daily ferries leaving for Mandraki, the main town and only port on Nisyros. Other ferry services to Nisyros are from: Astypalea - five hours, Chalki - 90 minutes, Kalymnos - three hours, and Rhodes - 41/2 hours. Some ferry services start from Piraeus, the port of Athens, calling at other Aegean islands on the way.
Accommodation on Nisyros consists mostly of family-run hotels and villas in Paloi and Mandraki. The local tourism office can be contacted on +30 22420 31204.