A hot tub is a large tub full of water used for hydrotherapy, relaxation or pleasure. Some have powerful jets for massage purposes. Hot tubs are sometimes also known as "spas" or by the trade name Jacuzzi. In contrast to a typical bathtub, a hot tub is designed to be used by more than one person at a time, with many models accommodating four or more people. Hot tubs are usually located outdoors, although they can be installed indoors. Another difference between baths and hot tubs is that soaps and shampoos are not used in wet-jetted hot tubs (although they can be used in air-jetted hot tubs). Home hot tubs are often closer in construction to standard bathtubs, while the construction of a public hot tub often has more in common with a swimming pool, of which it can be considered a type. History Main article: Bathing The earliest hot tubs were calderas in which hot stones were placed to heat the water. Therma in Ikaria has been a very popular place particularly for hydrotherapy ever since the 4th century B.C. The remains of wrecked marble bathtubs along with a prehistoric aqueduct that have been unearthed from this area bear ample testimony of the place's popularity in the ancient times. In 737 A.D., Japan's first onsen opened near Izumo, Shimane, and centuries later, the first ryokan (inns) were built, offering food, accommodations, and soaking tubs called ofuro. In ancient Rome, there were three types of baths: Baths at home (balnea), private baths (balnea privata), and public baths (balnea publica). The practice of bathing was so engrained that the Roman legions, during their long occupations in foreign lands, built their own baths at mineral and thermal springs in the newly conquered lands. Examples are found all over Europe. In the 1940s, hot tubs began to appear in the US, inspired by the Japanese ofuro. Hydrotherapy pumps were introduced by Jacuzzi. Fiberglass shell hot tubs appeared around 1970 and were soon superseded by cast acrylic shells. Water treatment Plumbing The plumbing of the hot tub consists of: A pressure system delivering water to the jets. A suction system returning water to the pumps. A filtration system: the plumbing has to incorporate a filter system to help clean the water. Some models use a separate small 24/7 filter pump while others use programmed settings of the main pumps. Induced air: The jets may