How far may a swimmer travel under water?

• The rule: "At least one part of the Athlete's body must break the surface of the water throughout the race, except that the Athlete may be completely submerged during the turn and for a distance of not more than 15 metres after the start and after each turn."
• The head: "By the 15 metre-mark, the Athlete's head must have broken the surface of the water."
• Where the mark is: the floats on each lane rope at 15 m from each end wall are a different colour from the others. Marks are also placed on both sides of the pool.
• In butterfly: the swimmer "is permitted one or more leg kicks and one arm pull under the water, which must bring the Athlete to the surface".
• Near the finish: once the head has passed the 5 m mark before the finish wall, the swimmer may be completely submerged while reaching for the wall.
• During a turn: the swimmer may be fully under water.

The kick used under water is the underwater dolphin kick, held in a streamline. The limit is the 15-metre mark.

A good underwater dolphin kick is faster than swimming on the surface. Left alone, races would be won under water, out of sight, by whoever could hold their breath longest. The limit keeps the stroke itself at the centre of the race and takes away a reason to stay down dangerously long.