The Caldera in Brief
Santorini is the rim of a flooded volcanic caldera. The cliffs of Fira and Oia, the island of Therasia and the small islet of Aspronisi are all fragments of the ring left after a series of eruptions, the largest being the Late Bronze Age eruption around the seventeenth century BC that buried the town of Akrotiri. In the centre of the caldera, two dark, younger islands have grown from repeated eruptions over the last two thousand years: Palea Kameni (Old Burnt Island) and Nea Kameni (New Burnt Island).
Nea Kameni was built up in several eruptive episodes, including 1707–1712, 1866–1870, 1925–1928 and 1939–1941, and its last small eruption was in 1950. The volcano is still active and is monitored by the Institute for the Study and Monitoring of the Santorini Volcano. Neither island is inhabited, and the only way to reach them is by boat.
Getting into the Caldera
Most visitors join an organised boat trip. The traditional wooden boats leave from the old port of Fira, below the town, which you reach by the cable car or by walking the long stepped path; other trips start from Athinios, Ammoudi below Oia, or the Vlychada marina on the south coast. Trips range from short tours of the two volcanic islands to day or sunset cruises that add Therasia and swimming stops.
When comparing trips, check exactly what is included: whether the boat stops at Nea Kameni long enough to walk to the crater, whether it goes into the bay at Palea Kameni for the warm springs, and whether lunch or transfers are included. Cheaper short tours usually cover the volcano and springs only; catamaran cruises often skip the crater walk entirely. A small cash entrance fee for the protected volcanic area is collected on Nea Kameni and is not always included in the boat price.
The Nea Kameni Crater Walk
Boats tie up at a small jetty on the north side of Nea Kameni. From there a gravel and cinder path climbs over black lava flows to the summit craters, a little over a hundred metres above the sea. The walk up takes most people around half an hour and the round trip on the island is commonly allowed about an hour to an hour and a half. You can walk around the crater rim and see steam and sulphur deposits around active vents, along with views back to the caldera cliffs.
It is not a difficult hike, but it is a real one. There is no shade, the dark rock absorbs heat, and the path is loose and uneven in places. Wear closed shoes with grip rather than flip-flops, carry water and a hat, and in July and August choose a morning departure. Stay on the marked paths; the surface around the vents is fragile and the area is a protected scientific site.
Swimming at the Palea Kameni Hot Springs
The springs are in a small bay on Palea Kameni. Boats anchor offshore and passengers jump in and swim a short distance to the inlet, where iron-rich water seeping from the volcano turns the sea a rusty orange-brown and noticeably warmer than the surrounding water. "Hot springs" overstates it: the water is warm rather than hot, and its temperature varies with the season and the tide.
You need to be a confident swimmer, since the boats cannot come close in and the water is deep. Wear an old or dark swimsuit, because the iron can stain light fabrics, and rinse off afterwards. Children and weaker swimmers can usually stay on board.
Aspronisi and Therasia
Aspronisi, whose name means "white island", is a tiny uninhabited islet on the south-western side of the caldera, between Therasia and the southern tip of Santorini. Its cliffs are capped by pale pumice and ash from the Bronze Age eruption, which gives it its colour and makes it one of the visible remnants of the old caldera rim. It has no facilities, and most visitors see it from the deck of a sunset or caldera cruise rather than landing.
Therasia, the larger island to the west, is inhabited and much quieter than Santorini. Some boat trips stop at its small port of Korfos, from where a stepped path climbs to the village of Manolas on the ridge; it can also be reached by local ferry.
Safety and Current Conditions
Santorini sits in an active volcanic and seismic area. In early 2025 an intense earthquake swarm in the sea between Santorini and Amorgos led to temporary restrictions, including closures of some caldera paths and ports. Conditions return to normal between such events, but you should check current local advice before travelling.
Wind is the more common reason a trip changes. Strong north winds in summer can cancel sailings or skip the swimming stop at Palea Kameni. If the volcano trip is a priority, schedule it early in your stay so there is time to rebook.
