Algar de Benagil: the collapsed dome, the opening, and the rules

The whole of this coast is the same soft, pale limestone, laid down when the Algarve was under a shallow sea and lifted since. It is porous, it dissolves, and the Atlantic works on it constantly from below while rain works on it from above.

Sea caves form where a weakness in the rock meets the waterline. The swell drives into the crack, compresses the air inside it, and takes the rock apart a grain at a time. Given enough centuries you get a chamber.

What makes Algar de Benagil different is the ceiling. It thinned from both directions until it could no longer carry itself, and a circular section of it dropped. What is left is a dome with a hole in the top, sand on the floor where the debris was ground down, and two openings to the sea.

It is still happening. Rockfall along this coast is routine, which is one reason the authorities have steadily tightened what you may do inside, and why every guide wears a helmet near the cliff faces.

The Portimão harbour master governs navigation here, and the current instruction is Edital 019/2024. It is a public document and it is unusually specific.

Prohibited outright: landing on or using the sand inside the cave; entering by swimming or on any unpowered flotation aid; entering when visibility is under a kilometre; scuba or free diving inside or in the approaches; navigation between sunset and sunrise; fishing in or near the cave.

Capacity inside: up to three motorised craft under twelve metres at the same time, or a single craft over twelve metres on its own. Non-motorised craft work in groups of six accompanied by one guide holding first-aid and basic life support certification, with up to seven of them inside at once, entering through the eastern arch.

Time inside: two minutes for motorised craft; the kayak or paddleboard group may not exceed eight.

Fines for operators run from €300 into the hundreds of thousands. That is why the skipper counts, why the guide moves the group along, and why nobody will make an exception for your photograph.

The opening is roughly circular and faces straight up, so the light through it moves across the chamber as the sun climbs. Early in the morning the column lands at the back, on the sand. Around the middle of the day it is on the water, and the whole cave turns pale green from underneath because the light is coming up off the bottom.

Later in the afternoon it slides off the water onto the western wall and the chamber goes noticeably darker. It is still worth seeing; it is simply not the photograph you had in mind.

For the classic shot — sand lit, water dark, the opening bright above — take the first departure of the day. That is also when the sea is flattest and when you are least likely to be queuing outside for a gap.

You cannot get into the cave on foot, but you can stand on top of it. From Benagil village a track climbs east out of the car park and runs along the clifftop; twenty minutes of easy walking brings you to the point where the collapsed roof opens at your feet.

It is unfenced. There is a rope in places and a sign, and every year people ignore both. The rock is undercut from below by the same sea that made the chamber, which is exactly why the roof fell in the first place.

From up there you look straight down through the hole onto the sand and the water, with boats turning inside like something in a model. It is a completely different view from the one on the postcards, it costs nothing, and most people who do the boat trip never bother with it.

The same path continues east as the Seven Hanging Valleys trail, past Praia da Marinha and on towards Albandeira. It is the best free half day on this coast.