robably be insufficient totake appropriate action。 the greater likelihood； however； is that we wouldn鈥檛 see any object鈥攅ven a et鈥攗ntil it was about six months away； which would be much too late。
shoemaker…levy 9 had been orbiting jupiter in a fairly conspicuous manner since 1929； but ittook over half a century before anyone noticed。
interestingly； because these things are so difficult to pute and must incorporate such asignificant margin of error； even if we knew an object was heading our way we wouldn鈥檛know until nearly the end鈥攖he last couple of weeks anyway鈥攚hether collision was certain。
for most of the time of the object鈥檚 approach we would exist in a kind of cone of uncertainty。
it would certainly be the most interesting few months in the history of the world。 and imaginethe party if it passed safely。
鈥渟o how often does something like the manson impact happen？鈥潯 asked anderson andwitzke before leaving。
鈥渙h； about once every million years on average；鈥潯aid witzke。
鈥渁nd remember；鈥潯dded anderson； 鈥渢his was a relatively minor event。 do you know howmany extinctions were associated with the manson impact？鈥�
鈥渘o idea；鈥潯 replied。
鈥渘one；鈥潯e said； with a strange air of satisfaction。 鈥渘ot one。鈥�
of course； witzke and anderson added hastily and more or less in unison； there wouldhave been terrible devastation across much of the earth； as just described； and pleteannihilation for hundreds of miles around ground zero。 but life is hardy； and when the smokecleared there were enough lucky survivors from every species that none permanentlyperished。
the good news； it appears； is that it takes an awful lot to extinguish a species。 the badnews is that the good news can never be counted on。 worse still； it isn鈥檛 actually necessary tolook to space for petrifying danger。 as we are about to see； earth can provide plenty of dangerof its own。
14 THE FIRE BELOW
灏忚txt澶╁爞
in the summer of 1971； a young geologist named mike voorhies was scouting around onsome grassy farmland in eastern nebraska； not far from the little town of orchard； where hehad grown up。 passing through a steep…sided gully； he spotted a curious glint in the brushabove and clambered up to have a look。 what he had seen was the perfectly preserved skull ofa young rhinoceros； which had been washed out by recent heavy rains。
a few yards beyond； it turned out； was one of the most extraordinary fossil beds everdiscovered in north america； a dried…up water hole that had served as a mass grave for scoresof animals鈥攔hinoceroses； zebra…like horses； saber…toothed deer； camels； turtles。 all had diedfrom some mysterious cataclysm just under twelve million years ago in the time known togeology as the miocene。 in those days nebraska stood on a vast； hot plain very like theserengeti of africa today。 the animals had been found buried under volcanic ash up to tenfeet deep。 the puzzle of it was that there were not； and never had been； any volcanoes innebraska。
today； the site of voorhies鈥檚 discovery is called ashfall fossil beds state park； and it has astylish new visitors鈥櫋enter and museum； with thoughtful displays on the geology of nebraskaand the history of the fossil beds。 the center incorporates a lab with a glass wall throughwhich visitors can watch paleontologists cleaning bones。 working alone in the lab on themorning i passed through was a cheerfully grizzled…looking fellow in a blue work shirt whomi recognized as mike voorhies from a bbc television documentary in which he featured。
they don鈥檛 get a huge number of visitors to ashfall fossil beds state park鈥攊t鈥檚 slightly inthe middle of nowhere鈥攁nd voorhies seemed pleased to show me around。 he took me to thespot atop a twenty…foot ravine where he had made his find。
鈥渋t was a dumb place to look for bones；鈥潯e said happily。 鈥渂ut i wasn鈥檛 looking for bones。 iwas thinking of making a geological map of eastern nebraska at the time； and really just kindof poking around。 if i hadn鈥檛 gone up this ravine or the rains hadn鈥檛 just washed out that skull；i鈥檇 have walked on by and this would never have been found。鈥潯e indicated a roofedenclosure nearby； which had bee the main excavation site。 some two hundred animalshad been found lying together in a jumble。
i asked him in what way it was a dumb place to hunt for bones。 鈥渨ell； if you鈥檙e looking forbones； you really need exposed rock。 that鈥檚 why most paleontology is done in hot； dry places。
it鈥檚 not that there are more bones there。 it鈥檚 just that you have some chance of spotting them。
in a setting like this鈥濃�攈e made a sweeping gesture across the vast and unvarying prairie鈥斺�測ou wouldn鈥檛 know where to begin。 there could be really magnificent stuff out there； butthere鈥檚 no surface clues to show you where to start looking。鈥�
at first they thought the animals were buried alive； and voorhies stated as much in anational geographic article in 1981。 鈥渢he article called the site a 鈥榩ompeii of prehistoric animals；鈥櫋♀�潯e told me； 鈥渨hich was unfortunate because just afterward we realized that theanimals hadn鈥檛 died suddenly at all。 they were all suffering from something calledhypertrophic pulmonary osteodystrophy； which is what you would get if you were breathing alot of abrasive ash鈥攁nd they must have been breathing a lot of it because the ash was feetthick for hundreds of miles。鈥潯e picked up a chunk of grayish； claylike dirt and crumbled itinto my hand。 it was powdery but slightly gritty。 鈥渘asty stuff to have to breathe；鈥潯e went on；鈥渂ecause it鈥檚 very fine but also quite sharp。 so anyway they came here to this watering hole；presumably seeking relief； and died in some misery。 the ash would have ruined everything。 itwould have buried all the grass and coated every leaf and turned the water into an undrinkablegray sludge。 it couldn鈥檛 have been very agreeable at all。鈥�
the bbc documentary had suggested that the existence of so much ash in nebraska was asurprise。 in fact； nebraska鈥檚 huge ash deposits had been known about for a long time。 foralmost a century they had been mined to make household cleaning powders like et andajax。 but curiously no one had ever thought to wonder where all the ash came from。
鈥渋鈥檓 a little embarrassed to tell you；鈥潯oorhies said； smiling briefly； 鈥渢hat the first i thoughtabout it was when an editor at the national geographic asked me the source of all the ash andi had to confess that i didn鈥檛 know。 nobody knew。鈥�
voorhies sent samples to colleagues all over the western united states asking if there wasanything about it that they recognized。 several months later a geologist named billbonnichsen from the idaho geological survey got in touch and told him that the ash matcheda volcanic deposit from a place called bruneau…jarbidge in southwest idaho。 the event thatkilled the plains animals of nebraska was a volcanic explosion on a scale previouslyunimagined鈥攂ut big enough to leave an ash layer ten feet deep almost a thousand miles awayin eastern nebraska。 it turned out that under the western united states there was a hugecauldron of magma； a colossal volcanic hot spot； which erupted cataclysmically every600；000 years or so。 the last such eruption was just over 600；000 years ago。 the hot spot isstill there。 these days we call it yellowstone national park。
we know amazingly little about what happens beneath our feet。 it is fairly remarkable tothink that ford has been building cars and baseball has been playing world series for longerthan we have known that the earth has a core。 and of course the idea that the continents moveabout on the surface like lily pads has been mon wisdom for much less than a generation。
鈥渟trange as it may seem；鈥潯rote richard feynman； 鈥渨e understand the distribution of matterin the interior of the sun far better than we understand the interior of the earth。鈥�
the distance from the surface of earth to the center is 3；959 miles； which isn鈥檛 so very far。
it has been calculated that if you sunk a well to the center and dropped a brick into it； it wouldtake only forty…five minutes for it to hit the bottom （though at that point it would beweightless since all the earth鈥檚 gravity would be above and around it rather than beneath it）。
our own attempts to penetrate toward the middle have been modest indeed。 one or two southafrican gold mines reach to a depth of two miles； but most mines on earth go no more thanabout a quarter of a mile beneath the surface。 if the planet were an apple； we wouldn鈥檛 yethave broken through the skin。 indeed； we haven鈥檛 even e close。
until slightly under a century ago； what the best…informed scientific minds knew aboutearth鈥檚 interior was not much more than what a coal miner knew鈥攏amely； that you could dig down through soil for a distance and then you鈥檇 hit rock and that was about it。 then in 1906；an irish geologist named r。 d。 oldham； while examining some seismograph readings from anearthquake in guatemala； noticed that certain shock waves had penetrated to a point deepwithin the earth and then bounced off at an angle； as if they had encountered some kind ofbarrier。 from this he deduced that the earth has a core。 three years later a croatianseismologist named andrija mohorovi？i鈥瞔 was studying graphs from an earthquake in zagrebwhen he noticed a similar odd deflection； but at a shallower level。 he had discovered theboundary between the crust and the layer immediately below； the mantle； this zone has beenknown ever since as the mohorovi？i鈥瞔 discontinuity； or moho for short。
we were beginning to get a vague idea of the earth鈥檚 layered interior鈥攖hough it really wasonly vague。 not until 1936 did a danish scientist named inge lehmann； studyingseismographs of earthquakes in new zealand； discover that there were two cores鈥攁n innerone that we now believe to be solid and an outer one （the one that oldham had detected） thatis thought to be liquid and the seat of magnetism。
at just about the time that lehmann was refining our basic understanding of the earth鈥檚interior by studying the seismic waves of earthquakes； two geologists at caltech in californiawere devising a way to make parisons between one ear