over four billion years temperature at tly  t estimates range from someto 13,000°F—about as  as the Sun.

    ter core is in many ood, tt it is fluid and t it is t of magnetism. t forward by E. C.

    Bullard of Cambridge University in 1949 t t of t makes it, in effect, an electrical motor, creating tic field. tion is t ting fluids in t somes in wires.

    Exactly  it is felt pretty certain t it is connected s being liquid. Bodies t don’t ance—don’t ism.

    e kno Eartic field cime to time: during t o times as strong as no it reverses itselfevery 500,000 years or so on average, t average ability. t reversal  750,000 years ago. Sometimes it stays put formillions of years—37 million years appears to be t stretc otimes it er as little as 20,000 years. Altoget 100 million years it self about t  est unansion in the geological sciences.”

    e may be going tic field  in t century alone. Any diminution in magnetism is likelyto be bad neism, apart from es to refrigerators and keeping ourcompasses pointing t al role in keeping us alive. Space is full ofdangerous cosmic rays t in tic protection ear tatters. ic field is o ts. teract icles in tmospo createtc knohe auroras.

    A big part of terestingly enoug traditionally ttle effort to coordinate ’s going oninside. According to Ss and geops rarely go to tings or collaborate on the same problems.”

    Perter demonstrates our inadequate grasp of terior t out s up, and it ary reminder of tations of our understanding tio