Here's what happens when we dive deep.

In the first 30 or so feet underwater, the lungs, full of air, buoy your body toward the surface, forcing you to paddle as you go down. You feel the pressure on your body double at 33 feet underwater. At this depth, the contracting air will shrink your lungs to half their normal size. As you keep diving, at about 40 feet, you enter a gravityless area in the water column that freedivers call the "doorway to the deep." Here, the ocean stops pulling you up the surface and begins pulling you down. You place your arms at your sides in a skydiver pose, relax, and effortlessly drift deeper.

At 100 feet, the pressure triples. The Master Switch kicks in harder. Your heart rate reflexively beats even slower. This will help you conserve oxygen, which will allow you to dive deeper for longer. Heart rates of freedivers below 100 feet can plummet below half their normal resting rates. Some divers have recorded heart rates as low as 14 beats per minute, about a third the rate of a person in a coma; some freedivers have even reported heart rates as low as seven beats per minute. (The average resting heart rate for most humans is between about 60 to 100 beats per minute.) According to physiologists, a heart rate this low can't support consciousness. And yet, somehow, deep in the ocean, it does.

Around 300 feet, a depth reached often by freedivers, the walls of your organs and vessels, working like pressure-release valves, allow the free flow of blood and water into the thoracic cavity. Your chest collapses to about half its original size. During a dive in 1996, Cuban freediver Francisco Ferreras-Rodriguez's chest shrank from a circumference of 50 inches at the surface down to 20 inches by the time he reached his target depth of 436 feet. The Master Switch shifts into overdrive.

As you ascend to 200 feet, 150 feet, 100 feet, the Master Switch slowly reverses its effects: The heart rate increases, and the blood that flooded into your thoracic cavity now floods back out into your veins and arteries and organs. Your lungs reinflate with air. You become a land animal again.