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In page Snorkeling:

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Generally shallow reefs ranging from 1 to 4 meters (3 to 13 ft) are favored by snorkelers. Enough water cover to swim over the top without kicking the bottom is needed, but isolated shallow structure can be approached from the sides. Deeper reefs can also be explored, but repeated breath-holding to dive to those depths limits the number of practitioners, and raises the bar on the required fitness and skill level. Risk increases with increased depth and duration of the breath-hold excursions from the surface.[citation needed]