INTO THE ROCK

The most dramatic section runs from Portimao west to Sagres, a stretch of about forty kilometres that includes some of the most photographed coastal scenery in Europe. The caves are the result of the sea exploiting weaknesses in the rock -- faults and fractures that the waves have widened over time into tunnels and chambers. Some are accessible only by boat. Others can be reached on foot at low tide.

The colour of the water inside the caves is the thing that photographs cannot adequately convey. It is a particular shade of turquoise -- lit from below by the sand and from above by the sky -- that has no equivalent in any other context. You will try to describe it to people when you get home and you will fail.

The standard way to see the caves is by boat, and the standard departure point is the Portimao harbour. The trips run from April to October, take between ninety minutes and three hours depending on how far west you go, and cost between fifteen and thirty euros per person.

The boats are small -- typically six to twelve passengers -- and they are driven by local fishermen who know every cave and arch on the coast. They will take you inside the caves, cut the engine, and let you sit in the silence while the turquoise water laps against the walls. They will also, if you ask, take you to the beaches that are only accessible by sea.

The best time to go is early morning, before the wind picks up and before the other boats arrive. Book the first departure of the day and you will have the caves largely to yourself.

Ponta da Piedade, outside Lagos, is the most spectacular section of the Algarve coast and the destination for the longer boat trips from Portimao. The headland is a labyrinth of golden rock towers, arches, and sea stacks rising from the water, with caves and grottos cut into the base of the cliffs.

The boat trip from Portimao to Ponta da Piedade and back takes about three hours. It is not a comfortable three hours -- the Atlantic swell can be significant -- but it is one of the most visually extraordinary experiences available in southern Europe. Bring sunscreen, a hat, and a willingness to be slightly damp.