The Benagil sea caves: what else is along this coast

The cliffs here are a marine limestone laid down in a warm shallow sea and lifted above the waterline since. It is soft enough to carve with a knife and full of joints and bedding planes — natural lines of weakness that water finds.

The Atlantic does the rest. Swell drives into a crack, compresses the air trapped at the back of it, and the pressure prises the rock apart. Repeat that a few million times and the crack becomes a tunnel; keep going and the tunnel becomes a chamber. Where two chambers meet you get an arch, and when an arch falls you get a stack standing offshore.

Every stage of that sequence is visible on this coast at the same time, often within a few hundred metres. That is the real reason the boat trips are good, and it is the part the photographs of one cave never convey.

Algar Seco. East of Carvoeiro, a shelf of limestone eaten into pillars, holes and windows. It is the one you can reach on foot, down a staircase from the clifftop, and it is free.

Praia da Marinha. Two arches standing in the water off one of the best-known beaches in Portugal. Most boat trips anchor here for the swim stop.

Praia da Albandeira. A smaller cove east of Benagil with its own arch, quieter than anywhere else on this list.

Praia do Carvalho. Reached through a tunnel cut into the rock. Small, sheltered, and on the itinerary of several private trips.

Ponta da Piedade. West of Lagos and a different character — tall stacks, narrow passages and clear green water. Trips from Lagos take it in on the way east.

The unnamed ones. Dozens between Benagil and Armacão de Pêra. Some go in twenty metres and stop, a few go through the headland. This is where a small hull earns its price.

The mistake is to think of these trips as transport to Benagil. On a small boat the cave is one stop of several, and on a good morning the skipper will take you into three or four others first.

What that looks like in practice: the hull noses into a gap barely wider than itself, the engine drops to idle, and the light goes green because it is coming through the water rather than the air. Some of these chambers are big enough to turn in. Some you reverse out of. A few go through the headland and come out on the other side, which is the trick every skipper saves for last.

None of it is on the itinerary because none of it can be promised. Whether a given grotto is enterable depends on the swell that morning and on the tide, and the same cave that swallowed the boat yesterday is a wall of white water today.

That variability is why the reviews for these trips are as good as they are. People who expected a shuttle to one photograph get four caves and a swim, and write accordingly.

Almost everyone on these trips is taking pictures, and almost all of them come back with the same two: a bright hole in a dark ceiling, and a blurred cliff.

Inside the cave. The dynamic range is brutal — a sunlit opening against a shadowed chamber. A phone will expose for the bright part and lose the rest. Tap to expose for the sand rather than the sky and let the opening blow out; that is the picture that looks like being there.

The grottoes. Green light coming up through the water is the thing worth catching, and it needs a steady hand more than anything else. Brace against the hull, not against someone else.

The coast. Morning light comes from the east and lands along the cliffs, which is why the outbound leg from Albufeira and the return leg from Lagos photograph better than the other direction.

Keep the strap on. Phones go over the side here every day, and the water is deeper than it looks.