Freediving
Free diving to great depth is especially dangerous, but any diver who is not well trained can panic, or otherwise fail to return to the surface before they use up the oxygen in their body.  They can lose consciousness from anoxia and drown. Professional free-divers can go in the water over 400 feet without any tools for breathing. But some free-divers die. On November. 17, 2013, Nick Mevoli blacked out after trying free-diving at a Bahamas competition. He never woke up again.