The Algar de Benagil collapsed some six thousand years ago when wave action eroded the Miocene limestone beneath a coastal meadow, leaving a cathedral dome open to the sky through a thirty-metre oculus. What remains is a fifty-metre chamber whose ribbed vault catches the morning sun in bands of ochre, sienna and chalk white, the colours shifting as tides fill and drain the crescent beach at its base. Fishermen knew the grotto for centuries but rarely entered; the coastline here offers no safe anchorage, and Atlantic swells funnel through the arch with unpredictable force. The cave entered the global imagination only in the 2010s, when aerial photographs revealed its geometry to resemble a collapsed eye socket in the cliffs—a comparison that drove visitation from fewer than two thousand annual visitors in 2012 to more than three hundred thousand by 2019. Portuguese authorities restricted access in 2021, banning private watercraft within two hundred metres and limiting entry to licensed operators departing from Albufeira, Portimão, and Lagos marinas. Benagil sits along a twelve-kilometre stretch of the central Algarve where erosion has sculpted more than forty named sea caves, arches and blow-holes into the sedimentary rock. The limestone here is unusually soft, deposited during a warm Miocene epoch when this coast lay beneath a shallow tropical sea. Fossilised mollusc shells and echinoid fragments remain visible in the cross-bedded strata exposed along the cliff faces. Geologists note that the entire coastline is retreating at an average of three to five centimetres per year, a rate that accelerates during winter storm cycles when wave energy exceeds ten thousand joules per square metre. The Benagil dome itself shows fresh fracture lines along its northern flank, and monitoring stations installed in 2023 record subsidence of up to two millimetres annually. Access restrictions reflect both overcrowding and structural fragility; the Portuguese Institute for Sea and Atmosphere now limits simultaneous occupancy to fifteen visitors and prohibits landing on the interior beach between October and March, when swells regularly exceed two metres. Most benagil cave tour from Albufeira marina departures use rigid inflatable boats capable of navigating the narrow entrance even in moderate chop, though catamaran operators offer a steadier platform for passengers prone to seasickness. Morning departures between seven and ten benefit from calmer conditions and oblique sunlight that illuminates the dome's eastern wall, casting the oculus in sharp relief. Afternoon tours often include dolphin sightings along the fifteen-kilometre transit from Albufeira; common and bottlenose dolphins frequent the coastal shelf where upwelling currents concentrate sardine and mackerel shoals. The return leg typically detours past Praia da Marinha, Arco da Albandeira, and Gruta do Capitão, smaller formations that lack Benagil's fame but offer close-up views of the honeycombed erosion patterns and nesting sites used by yellow-legged gulls and European shags. Tour operators prohibited from landing at Benagil circle the interior for three to five minutes, allowing passengers to photograph the vault and observe the tidal scour marks that stripe the walls at half-metre intervals, a visual record of winter storm surges.