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Underwater swimming has unique features of breathing apparatus (SCUBA), thermal protective gear, and fins. The energy cost of underwater swimming is determined by the drag while swimming and the net mechanical efficiency. These are influenced by the cross-sectional area of the diver and gear and the frequency of the leg kick. The speeds that divers can achieve are relatively low, thus the ˙VO2 increases linearly with values of˙VO2·d-1 of 30-50 l·km-1 for women and men, respectively. Diving experience had little effect on ˙VO2 for women; however, male divers with experience had lower ˙VO2 than beginners. The location and density of the gear can alter the diver's attitude in the water and increase the energy cost of swimming by 30% at slow speeds. The type of fin used has an effect on the depth and frequency of the kick, thus on drag and efficiency, with a range of ˙VO2 from 25 to 50 l·km-1. A large flexible fin had the lowest energy cost and a large rigid fin the highest. Adding extra air tanks or a dry suit increased the cost of swimming by 25%. The energy cost of underwater swimming is influenced by gender, gear and its placement, fin type, and experience of the diver.