paragraph: New Horizons: Exhausted after the Civil War, Spain remained on the sidelines during World War II and began to recover economically under the oppressive, law-and-order regime of Franco. There had been a foretaste of elite foreign tourism in the 1920s, but it was the late 1950s when the rest of Europe began sun-seeking pilgrimages to Spain. Tourism exploded into an annual southern migration, transforming the Spanish economy, landscape, and society. Eager to capitalize, the country poured its soul into mass tourism, which triggered a rash of indiscriminate building on the southern and eastern coastlines, with scant regard for tradition or aesthetics. But after so many years closed off from the rest of Europe, of equal significance was the injection of foreign influences into Franco's once hermetically sealed Spain. Mallorca and Menorca in particular saw explosive growth in tourism; by the 1970s, the Balearics were one of Europe's most popular holiday destinations. Franco named as his successor the grandson of Alfonso XIII, who was enthroned as King Juan Carlos I when the dictator died in 1975. To the dismay of Franco diehards, the king brilliantly managed the transition to democracy, then stood back to allow it full rein, even intervening during a brief attempt at a military coup. After many years of repression, new freedoms and autonomy were granted to Spanish regions, including the Balearics, and their languages and cultures enjoyed a long-desired renaissance. More a part of Europe than ever before, Spain joined the European Community (now European Union) in 1986, giving further boost to a booming economy. The tourist industry continued to expand, and though it became one of the top two income earners in Spain, a realization that unrestricted mass tourism was leading to damaging long-term consequences also began to grow. By the late 1990s, a new emphasis on quality and, especially in the Balearics, on safeguarding the environment had finally taken root—too late for many environmentalists, but hopefully still in time to preserve much of the natural beauty and unique character of the Las Islas Baleares. 
question: When did tourism in Spain explode into an annual southern migration?
answer: The late 1950's