A visit to the caldera is like stepping into a science fiction movie, cows grazing amid strange coloured rocks in what resembles a lunar landscape.
The largest crater is Stefanos, where you can examine the multicoloured 100°C fumaroles, listen to their hissing and smell the sulphurous vapours. The surface is soft and hot, making sturdy footwear essential. A track leads to the wilder craters of Polyvotis and Alexandros. Take great care here as it's unsupervised, not fenced and fumaroles are around the edge.
If you're staying on the island, get here by 11am to avoid the busloads of day troopers that arrive from Kos.