The Algar de Benagil formed over six million years as Atlantic storms carved a keyhole breach through Jurassic limestone, creating a natural rotunda 25 metres wide with an oculus admitting a single column of light. Local fishermen sheltered here through the 19th century, dragging boats onto the sand beach inside when westerly gales swept the coast. The dome remained inaccessible except by sea until kayak outfitters began running guided paddles from Albufeira in the early 2000s, transforming an obscure fishing refuge into one of Europe's most photographed sea caves. Today the Benagil grotto anchors a seven-kilometre stretch of karst coastline between Praia da Marinha and Carvoeiro, where erosion has sculpted more than 40 navigable caves, natural bridges, and blowholes into the ochre cliffs. Geologists classify the formations as solution caves—seawater dissolved calcium carbonate along vertical fractures, then wave action hollowed horizontal chambers. The cathedral cave's golden colour comes from iron oxide in the limestone strata, intensified by afternoon sun filtering through the ceiling aperture. Kayak expeditions depart Albufeira Marina between dawn and dusk, covering eight to twelve nautical miles depending on swell and wind. Guides time departures around tidal windows—low tide exposes gravel beaches inside the caves, while spring tides push water to within two metres of vault ceilings. Atlantic bottlenose dolphins patrol the offshore banks year-round, and autumn migrations bring common and striped dolphins within 500 metres of the cliffs. The paddle route threads through Praia da Marinha's twin arches, skirts the collapsed amphitheatre at Gruta do Capitão, and beaches at Benagil's interior strand for twenty minutes beneath the dome. Outfitters limit group sizes to twelve kayaks to reduce congestion inside the chamber, where motor vessels are prohibited within a 200-metre exclusion zone. The benagil cave kayak tour from albufeira sector now operates under municipal licensing rules introduced in 2024, capping daily kayak launches at 180 and requiring certified ocean guides for all departures. Most operators offer semi-rigid boat alternatives for visitors uncomfortable with three-hour paddles, though these vessels cannot enter the cathedral cave's shallow interior. Morning launches between 07:30 and 09:00 avoid peak boat traffic and offer the calmest sea conditions, particularly during summer's afternoon thermal winds.