A great white shark can smell blood in a radius of approximately 1/3 mile or "as low as 1 part per 25 million." Not all sharks have this strong of a sense of smell; however, all sharks do have a sense of smell that is superior to many animals.
Sharks smell using nares, which are holes on the bottom of their long snouts. They need these incredible noses in order to find their next food source by not only determining what food is there, but also how to get to it. Interestingly, sharks are also able to sense small electrical fields due to the pores that are on their heads. The hammerhead shark uses its head to find food underneath the sand by sensing for the electrical currents that the prey animals give off.
Sharks are also able to use their lateral line, which is an organ on the side of their body, to find out where there is movement. This helps the shark to detect prey as well as possible predators. However, the shark is typically at the top of the food chain and does not need to worry about predators. Sharks are carnivorous in nature and are also the largest fish in the world.