Great hammerhead sharks can grow as long as 18 feet (5.5 meters) and weigh a thousand pounds (450 kilograms)!
Scientists have different ideas about why the sharks have such unusual heads. Some say that the hammer-shaped head helps the sharks swim better. Others say the head improves the sharks' vision.
One shark in this picture (bottom) is wearing a National Geographic Crittercam. Crittercams include video cameras and other information-gathering devices. These can be attached to animals without hurting them. Crittercams help scientists study the animals that wear them.