Lying 4,000 miles beneath the surface and with temperatures nearly as hot as the Sun’s, the inaccessibility of the planet’s center has long been a problem for researchers. In this program, scientists who have journeyed deep within the Earth explain how the core is studied and what findings reveal about the way the planet was formed. Incredibly, the malfunctioning of the Hubble telescope also led experts to a better understanding of the interior, and seismologists detonating explosives make contributions as well. But in Tokyo, geophysicists who have re-created conditions at the core are finding evidence that it is cooling faster than previously believed, with serious implications for life on Earth. A BBC Production. (59 minutes)