" At three hundred feet, we are profoundly changed. The pressure at these depths is nine times that of the surface. The organs collapse. The heart beats at a quarter of its normal rate, slower than the rate of a person in a coma. Senses disappear. The brain enters a dream state. At six hundred feet down, the ocean’s pressure—some eighteen times that of the surface—is too extreme for most human bodies to withstand. Few freedivers have ever attempted dives to this depth; fewer have survived. Where humans can’t go, other animals can. Sharks, which can dive below six hundred and fifty feet, and much deeper, rely on senses beyond the ones we know. Among them is magnetoreception, an attunement to the magnetic pulses of the Earth’s molten core. Research suggests that humans have this ability and likely used it to navigate across the oceans and trackless deserts for thousands of years. Eight hundred feet down appears to be the absolute limit of the human body. "

― James Nestor , Deep: Freediving, Renegade Science, and What the Ocean Tells Us about Ourselves