Swimming pools date back to the bathhouses in the palaces of ancient Greece. These bathhouses were elaborate spas, complete with steam rooms, saunas, and large pools. Swimming was a popular pastime even among those who did not have access to bathhouses; many swam in the rivers, oceans, and the lakes of the world. The plagues of medieval Europe made people cautious about swimming in unclean waters, but soon swimming regained popularity. Swimmers swam with their heads above water in a style developed when many people were still afraid of water contamination. This swimming style changed in the mid-1800s when Native Americans introduced an early version of the modern "crawl." Swimming in natural spring waters was even recommended as a health benefit, inspiring hospitals and spas to develop around hot springs.
The first modern Olympics held in 1896 in Athens, Greece, featured swimming as one of the nine competitions. Swimming as both a sport and a pastime has continued to develop along with the technology of pool maintenance. By the 1960s, the National Swimming Pool Foundation had evolved to support research in pool safety and the education of pool operators. Today, the U.S. swimming pool cleaning services industry generates approximately $3 billion in revenue and consists of more than 51,000 businesses, according to the market research group IBISWorld.