Via National Geographic:

A diver makes a slow decent into a vortex of 50,000 farmed salmon in British Columbia, Canada. As scuba divers sink deeper underwater, the weight of the water above them creates pressure. As the diver resurfaces, his body decompresses, and extra nitrogen escapes into the bloodstream, where it is carried to the lungs for excretion. If a diver surfaces too fast, bubbles can form in the blood and tissues, causing the bends.