Swim into a sea cave you can only reach at low tide

The cliff at Benagil looks solid until you're right against it, then a gap appears, barely wider than a doorway. Swim through at low tide, when the sea eases back from the sandstone, and the passage opens into a hollow chamber roofed by a collapsed dome, sunlight spilling through the hole into water the colour of green glass. Check tide tables before you go; the window is about two hours either side of low water, and the entrance narrows to nothing at high tide. Currents along this stretch of the Algarve run strong, so most swimmers go with a guide or pick a flat, windless morning. What stays with you is the sand floor, dry and pale, sitting under open sky in the middle of the sea.