The simplest true sentence about Playa de las Teresitas is that the sand is not from Tenerife. The island's north coast is volcanic, and the beaches it makes on its own are black or dark grey. The pale gold sand on Las Teresitas came from a desert on the African mainland, and it is there because a city decided it should be.

The brief. In 1953 the Santa Cruz de Tenerife council voted to build itself a proper bathing beach. The capital sat on a working volcanic coast with no good sand of its own, and the council wanted an amenity — a wide, golden, calm-water beach close to the city. It took years of planning and approvals before anything was built. The project that resulted was not a restoration of a natural beach; it was the construction of one from scratch, to specification.

The engineering. Two problems had to be solved: the Atlantic, and the sand. The Atlantic was handled first. From 1968 a stone breakwater was built about 150 meters offshore and run for roughly a kilometer, set to break the open swell and turn the exposed bay into a sheltered lagoon. Without it, any sand placed here would be dragged straight back out to sea. With it, the water inside shelves gently and stays flat — the reason Las Teresitas became the family beach of the capital rather than a surf coast.

The sand. Then came the fill. In 1971 the council took out a 50-million-peseta loan and bought 150,000 cubic meters of sand, weighing about 270,000 tons, from the El Aaiún region of what was then Spanish Sahara, the Spanish colony on the African coast to the south. The sand was packed into roughly 5 million sacks and carried across to Tenerife aboard a ship called the Gopegui, operated by the phosphate company Fosfatos de Bucraa. Over the first half of 1973 it was spread across the bay behind the new breakwater. The beach opened to the public on 15 June 1973.

The upkeep. A built beach is a maintained beach. The sea still works at the sand, and in 1998 the council added 2,800 more tonnes of Saharan sand to replace what had been lost, at a cost of 400 million pesetas. The gold color that reads as natural in every photograph is, in fact, a managed stock of imported material, replenished as it depletes — and, in the early years, accompanied by some unintended imports: red ants and scorpions carried over in the sand.

The honesty of it. Las Teresitas is sometimes described as if it were a found wonder, a tropical beach improbably attached to a volcanic island. It is better understood as good civic engineering: a city looked at a coast that would not give it the beach it wanted, and built that beach anyway, out of a breakwater and a desert shipped in sacks. The result is genuinely pleasant — 1,300 meters of warm gold sand, flat clear water, the dark Anaga ridges behind. What it is not is natural. The most interesting thing about the beach is exactly the thing the postcard leaves out: that it is a product, dated 1973, with a purchase order behind it.