Ride the Tide, Playa de Las Catedrales, Cantabria, Spain.
The caves and arches the erupt from the sand along Playa de Las Catedrales, also known as Praia de Augas Santas, or the "Beach of the Holy Waters" are best experienced at low tide. At high tide you'd need a sturdy skiff, though you'd still be scuttled against the ancient rocks.
We explored caves, archways, sandy corridors and sink holes as we wandered further along the beach, wholly engrossed in the scenery to the point that we didn't realize the tide was turning. We made our return to high ground just as the arches were swallowed up by the ocean, as if they had never been there at all.