Hot tubs, also popularly called jacuzzis, are engineered tubs or pools of hot water used for relaxation. Located either indoors in specialized rooms or outdoors in backyards, roofs or terraces, hot tubs are sized to seat multiple people simultaneously with various seating configurations and angles with integrated jet streams. The water in a hot tub is not changed with each used, but sanitation methods similar to those used for swimming pool filtration are used. Calderas, where hot stones were placed to heat the water, were the earliest form of hot tubs. Today, hot tubs are typically heated by an electric or natural gas heater, though they continue to be found at natural hot springs.
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