The Complete Handbook of Diving & Snorkeling

Chapter 1: Understanding Diving & Snorkeling

Diving and snorkeling are fundamentally about moving your body through water while breathing—a simple enough goal that requires solving multiple biological and mechanical problems simultaneously. Before you can understand why certain equipment exists or what specifications matter, you need to understand the physical reality you're trying to overcome.

Why Human Bodies Need Equipment Underwater

A human body is not naturally adapted to stay underwater. Your eyes focus differently in water than air; your ears receive pressure that can damage tissue; your lungs hold only about six liters of air; you lose heat rapidly because water conducts it away forty times faster than air does; and below about 40 feet, the gases you breathe begin to poison you. None of these problems exist on land. All of them become urgent problems underwater, and all of them are solved (or managed) by equipment.

At the surface, your lungs sit in a low-pressure environment. Atmospheric pressure at sea level is about 14.7 pounds per square inch, or one atmosphere. Every 33 feet you descend in salt water adds another atmosphere of pressure. At 33 feet, you're at two atmospheres. At 99 feet, you're at four atmospheres. This is not a minor inconvenience—it directly affects how your body behaves, how equipment functions, and which gases you can safely breathe.

When you descend, water pressure squeezes your rib cage, compressing the air in your lungs. If you hold your breath while ascending—a reflex many people instinctively want to do—that