The Algar de Benagil is a sea cave with a near-perfect round hole in its roof, opening onto a small beach hidden inside the cliff. That combination — a domed chamber, a natural skylight and an interior beach — is what makes it one of the most photographed spots on the Algarve. It is also a textbook example of how soft coastal limestone is reshaped by the sea.
The rock: Miocene limestone
The golden cliffs between Carvoeiro and Albufeira are made of limestone laid down under a warm, shallow sea during the Miocene epoch, very broadly on the order of twenty million years ago. Limestone is largely calcium carbonate, built from the shells and skeletons of marine life, and it has two properties that matter here: it is relatively soft, and it dissolves slowly in slightly acidic water. Both make it easy for weather and waves to carve.
How the cave was carved
Two processes worked together. Rainwater, mildly acidic, seeped into cracks and joints in the rock and slowly widened them — the same karst dissolution that forms caves inland. At the coast, the Atlantic added brute force: waves drove into weaknesses at the base of the cliff, compressing air and water into the joints and prising the rock apart. Over many thousands of years those weaknesses grew into sea caves running back into the headland.
The skylight
Benagil’s defining feature is the circular opening in its roof, known locally as the ‘olho’, or eye. It formed when the roof of the chamber, thinned from below by the sea and from above by rainwater sinking through the rock, finally collapsed — a feature geologists call a collapse doline. The debris was cleared by the waves, leaving a clean shaft that drops daylight straight onto the sand below. At the right time of morning the light pours in like a spotlight.
The arches and the beach
The chamber opens to the sea through two arches at water level, which is how boats and kayaks get in. Between them, sheltered from the open Atlantic, waves and currents have piled up a small pocket of sand — the interior beach. Because it sits inside the cliff, it is calm and shaded for much of the day, lit mainly by the shaft from the skylight above.
Reading the cave walls
Look closely at the walls inside and you can read the cave’s history in the rock. The limestone is layered in bands, each one a period of deposition on the ancient sea floor, tilted gently and stained ochre and grey by iron and weathering. The smoother, scalloped hollows lower down are cut by the sea; the rougher, pitted rock higher up is dissolved by rainwater and salt spray. Together they show the two forces — water working from below and water working from above — that shaped the whole chamber over millions of years.
Benagil in its coastline
Benagil is the most famous of dozens of sea caves, arches and stacks along this stretch of coast. The same limestone forms the arches at Albandeira, the sculpted cliffs of Praia da Marinha and the grottoes off Carvoeiro and Lagos. Seeing several on one trip is part of why the boat and kayak tours here range so widely, from a quick hour at Benagil to half-day cruises along the whole coast.
See the cave for yourself — compare every Benagil Cave tour, or read the visiting guide for the rules and the best time to go.