The 9 o’clock bus from Barcelona to the Costa Brava resort of Tossa de Mar is packed, mostly with young Asian women. Seventy-five minutes later they make straight for the Forat del Dimoni (Devil’s Hole), a small gap in the old city wall, where they queue to take each other’s pictures in a variety of poses. After that they post their pictures, take a stroll around town, eat lunch, then take the bus back to the city. “It all started two or three months ago,” says Gregori Martínez of the Tossa de Mar tourism department. “Most of them are young...