Why is it so difficult to breathe when snorkeling at a depth of 1?

Because your lungs get inflated like a balloon they become compressed when submerged in water and the air within is compressed. Air will not itself flow from one region of low pressure into a region of higher pressure. The diaphragm in one's body reduces lung pressure to permit breathing, but limit is strained when nearly 1m below the water surface. It is exceeded at more than 1,m.

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