Algar de Benagil is a sea cave lit from above. A roughly circular hole in its domed roof drops daylight onto a crescent of golden sand sealed inside the rock — a beach no path from land can reach. Two arched mouths face the sea, so the chamber is never fully dark, and the water inside carries the same pale green as the shallows outside. Atlantic swell drove into the soft limestone of this coast and hollowed out the dome, while rainwater seeping down from the clifftop dissolved the same rock from the other side, until the thinning ceiling gave way. Portuguese calls a shaft like that an algar, and the name stuck to the cave.
You can only meet it from the water, and since August 2024 you can only look. Maritime rules now close the cave to swimmers and floating devices, and no boat or kayak may land on the sand. Licensed motorboats get a two-minute turn inside, guided kayaks and paddleboards a little longer. So you drift beneath the dome instead of standing under it: engine cut, voices bouncing off the layered walls, the beach a few metres away and out of reach. Around midday in the high-sun months the sun climbs steep enough to shine straight down the shaft and lay a disc of light on the sand. At other hours it slants in through the arches and the chamber turns amber. Dozens of caves pit the cliffs between Lagos and Albufeira, and this is the one whose roof opened. From the clifftop above, the Seven Hanging Valleys trail reaches the fenced rim of the same hole, and you see almost nothing of the beach below.