Most shark species are especially attracted to the smell of blood. According to National Geographic, the great white shark can detect just one drop of blood in 25 gallons of water and can even detect minute amounts in the ocean from up to 3 miles away.
A shark's exact ability to smell blood is heightened by the speed of water currents as the scent may dissipate much sooner in slower-moving currents. While the presence of blood in the water indicates the presence of wounded prey and an easy meal, it does not necessarily mean an attack is imminent. If the shark is not hungry, it is unlikely to look for the source.