Only a few species of shark are dangerous to humans. Out of more than 480 shark species, only three are responsible for two-digit numbers of fatal unprovoked attacks on humans: the great white, tiger and bull; however, the oceanic whitetip has probably killed many more castaways which have not been recorded in the statistics. These sharks, being large, powerful predators, may sometimes attack and kill people; however, they have all been filmed in open water by unprotected divers. The 2010 French film Oceans shows footage of humans swimming next to sharks in the ocean. It is possible that the sharks are able to sense the presence of unnatural elements on or about the divers, such as polyurethane diving suits and air tanks, which may lead them to accept temporary outsiders as more of a curiosity than prey. Uncostumed humans, however, such as those surfboarding, light snorkeling or swimming, present a much greater area of exposed skin surface to sharks. In addition, the presence of even small traces of blood, recent minor abrasions, cuts, scrapes or bruises, may lead sharks to attack a human in their environment. Sharks seek out prey through electroreception, sensing the electric fields that are generated by all animals due to the activity of their nerves and muscles.