The roof of Algar de Benagil collapsed on its own, and nobody recorded when. The sea cave sits inside soft Miocene limestone laid down roughly twenty million years ago, and the circular opening in its dome — the feature every photograph fixes on — is simply the place where erosion won. Locals call it the olho, the eye. Sunlight drops through it onto a crescent of sand at a steepening angle from mid-morning onward. The coastline that carries visitors there is equally accidental. Between Lagos and Carvoeiro, the Atlantic has cut some twenty kilometres of ochre cliff into stacks, blowholes and grottoes: Ponta da Piedade at the western end, Praia da Marinha and Algar Seco further east. Fishermen worked these waters for centuries from Marina de Lagos, 8600-315 Lagos, Portugal, long before any operator advertised benagil & dolphin watching from lagos as a half-day outing. The rock is porous enough that whole galleries have opened and closed within living memory, which is why skippers read the swell before committing to an entrance. What makes the route matter now is biological as much as geological. The waters off the Barlavento coast host resident and migratory cetaceans — common dolphins in large feeding pods, bottlenose dolphins closer to the shelf edge, and occasionally minke whales. Marine biologists sail on several of the catamaran and speedboat departures, logging sightings that feed regional monitoring records. The cave itself carries no entrance fee at all: entry costs 0 EUR, and operators charge for boat access rather than for the grotto. Portuguese authorities restrict swimming and unaccompanied landings inside Benagil, so licensed craft have become the practical means of seeing it. Architecturally, the interest is subtractive rather than built. The dome rises perhaps twenty metres; two sea-facing arches admit light from the south while the oculus admits it from above, and the intersection of the two produces the banded, cathedral-like effect that draws photographers from across the Algarve. Nearby grottoes — Cathedral Cave, the Elephant, Boneca — repeat the pattern at smaller scale. Modern benagil & dolphin watching from lagos tours generally trace this whole sequence rather than isolating one cave, pairing the geology of the cliffs with open-water time spent watching dorsal fins break a flat morning sea. Understood this way, the coast reads as a single continuous record of water working against soft stone.