Coelacanths are giant primitive fish that scientists knew from fossils, but believed had died out millions of years ago. Then in 1938, a living specimen of the "extinct" species was caught off South Africa. Now scientists in submersibles have studied coelacanths living on the sea-bed, off the island of Madagascar. Coelacanths are giant, blue-scaled fish with muscly fins which they may use to perch on the ocean bottom.