For humans and sharks to coexist, there must be a way to avoid attacks without violence. Since people began exploring the oceans, researchers have been racing to find the perfect repellent. Around the beginning of World War II, scientists were asked to discover ways to alleviate the danger to soldiers stranded in water after shipwreck or plane ejection — a task that has led to some of the strangest and most prolific shark studies in history. Over time, researchers have developed electrical, chemical and even natural shark deterrents, though a foolproof repellent is yet to be found. Even with scientists' most intelligent tricks, we're still unable to guarantee the masking of human presence in the water from these animals, which exist in nearly 520 different species around the world.