Many of the superhumans on Earth owe their powers to the "metagene", a genetic feature of unknown origin, which causes some people to develop superpowers when exposed to dangerous substances and forces. Others owe their powers to magic, genetic manipulation (or mutation) or bionics ("see below"). A large power gap resides between most superheroes and civilians. Still others owe their powers to not being human at all ("see" races, "below"). There are also superheroes and supervillains who possess no superhuman powers at all (for example Batman, Robin, Green Arrow or Speedy), but rival their effectiveness with specialized equipment or "to the absolute limit of human potential" training in special skills, such as martial arts.