Question

why must scuba divers ascend to the surface at a controlled rate after a deep sea dive?

Answer 1

At exceptionally deep dives additional nitrogen dissolves in the blood stream of the diver. A slow ascent allows the elemental N2 to escape from the blood stream and be exhaled through the lungs. This "cleansing" process doesn't take place immediately; therefore, the ascent must be slow to allow this exchange time to take place.