100 feet (30 meters) is the magic number. It’s the depth where the historic shipwrecks rest, the large pelagic life appears, and the ocean feels truly vast. But diving deep isn’t just about swimming further down; it fundamentally changes the physics of your dive. From shrinking wetsuits to the intoxicating effects of nitrogen narcosis, here is exactly what happens to your body and brain when you cross the recreational depth threshold, and why proper training is absolutely essential.