Sometimes they decide they no longer need to breathe through their mouthpiece. The precise symptoms and the depths to which the symptoms appear vary with each individual and with each dive. Diving below 100 feet requires special skills and is dangerous. Returning to the surface reduces the nitrogen content and reduces the symptoms. If one atmosphere equals about 14.6 pounds per square inch pressure, and the pressure increases 1 atmosphere for every 10 meters of depth.
Divers limiting the time and depth of their dives can avoid nitrogen narcosis.
Coming to the surface in stages with a pause at each stage allows the nitrogen to diffuse out of the blood. Pressures adapted from Project For Sea by Jim Kolb.