The Santorini caldera holds 1,200 feet of seawater where a volcanic crater collapsed thirty-six centuries ago, leaving a crescent of cliffs that drop vertically into the Aegean. What remains is a drowned basin ringed by lava strata — rust, charcoal, ochre — exposed in bands visible from any point on the water. The eruption that formed it buried Akrotiri under pumice and may have inspired the Atlantis myth; the caldera itself is the largest active volcanic system in the southern Aegean, its submerged floor still venting steam. Modern Santorini wraps around three sides of this basin, white-washed villages perched on the rim where the cliffs are steep enough to require cable cars. A sunset sailing tour Santorini embarks from Vlichada Marina on the island's southern coast, away from the cruise-ship crowds that funnel into Fira. Catamarans depart in mid-afternoon, tracking northwest along the caldera's inner wall past the black-sand beaches of Perissa and Kamari. The route passes Nea Kameni, the dark volcanic islet at the basin's center, and Thirasia, the western fragment of the original island. Most cruises anchor near the hot springs off Nea Kameni for a swimming stop, then position off Oia for the final hour of daylight. The water here is deep — soundings exceed 1,300 feet — and holds temperature year-round, making it swimmable from April through October. The caldera's orientation amplifies the sunset effect. Oia sits at the northern tip of the crescent, facing due west across open water to Thirasia. As the sun drops toward the horizon, the cliffs behind Oia catch the last direct light, their strata glowing amber against the darkening sky. Viewed from a boat, the effect is geometric: vertical cliffs, horizontal water, and the sun's trajectory bisecting both. The volcanic rock absorbs and reflects wavelengths differently than limestone or granite, shifting from white to peach to crimson as the angle changes. Most sunset sailing tour Santorini tickets include this northern positioning, holding station offshore while the light fades and the village lights begin to appear along the clifftops above.