Spearfishing
You take your diving equipment, a lot of adventurous spirit and a deep breath and you dive. From that point on there is nothing else in the world apart from you and the nature. A few hours of emptying your mind from any stressful thought and becoming one with the environment.
Spearfishing is more than a sport in my opinion. It is a way of viewing things. It is about being respectful towards the nature and your personal limits. Even though it is an extreme sport and it does indeed drive you to your limits, the feeling of freedom and self control that gives are simply not comparable. No words to explain. Just try it and you'll see!
Some interesting spearfishing links: