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hinking was that if they could understand the nature of the rocks inside the earth； they mightbegin to understand how they interacted； and thus possibly be able to predict earthquakes andother unwele events。
the project became known； all but inevitably； as the mohole and it was pretty welldisastrous。 the hope was to lower a drill through 14；000 feet of pacific ocean water off thecoast of mexico and drill some 17；000 feet through relatively thin crustal rock。 drilling froma ship in open waters is； in the words of one oceanographer； 鈥渓ike trying to drill a hole in thesidewalks of new york from atop the empire state building using a strand of spaghetti。鈥�
every attempt ended in failure。 the deepest they penetrated was only about 600 feet。 themohole became known as the no hole。 in 1966； exasperated with ever…rising costs and noresults； congress killed the project。
four years later； soviet scientists decided to try their luck on dry land。 they chose a spot onrussia鈥檚 kola peninsula； near the finnish border； and set to work with the hope of drilling toa depth of fifteen kilometers。 the work proved harder than expected； but the soviets weremendably persistent。 when at last they gave up； nineteen years later； they had drilled to adepth of 12；262 meters； or about 7。6 miles。 bearing in mind that the crust of the earthrepresents only about 0。3 percent of the planet鈥檚 volume and that the kola hole had not cuteven one…third of the way through the crust； we can hardly claim to have conquered theinterior。
interestingly； even though the hole was modest； nearly everything about it was surprising。
seismic wave studies had led the scientists to predict； and pretty confidently； that they wouldencounter sedimentary rock to a depth of 4；700 meters； followed by granite for the next 2；300meters and basalt from there on down。 in the event； the sedimentary layer was 50 percentdeeper than expected and the basaltic layer was never found at all。 moreover； the world downthere was far warmer than anyone had expected； with a temperature at 10；000 meters of 180 degrees centigrade； nearly twice the forecasted level。 most surprising of all was that the rockat that depth was saturated with water鈥攕omething that had not been thought possible。
because we can鈥檛 see into the earth； we have to use other techniques； which mostly involvereading waves as they travel through the interior。 we also know a little bit about the mantlefrom what are known as kimberlite pipes； where diamonds are formed。 what happens is thatdeep in the earth there is an explosion that fires； in effect； a cannonball of magma to thesurface at supersonic speeds。 it is a totally random event。 a kimberlite pipe could explode inyour backyard as you read this。 because they e up from such depths鈥攗p to 120 milesdown鈥攌imberlite pipes bring up all kinds of things not normally found on or near thesurface： a rock called peridotite； crystals of olivine； and鈥攋ust occasionally； in about one pipein a hundred鈥攄iamonds。 lots of carbon es up with kimberlite ejecta； but most isvaporized or turns to graphite。 only occasionally does a hunk of it shoot up at just the rightspeed and cool down with the necessary swiftness to bee a diamond。 it was such a pipethat made johannesburg the most productive diamond mining city in the world； but there maybe others even bigger that we don鈥檛 know about。 geologists know that somewhere in thevicinity of northeastern indiana there is evidence of a pipe or group of pipes that may be trulycolossal。 diamonds up to twenty carats or more have been found at scattered sites throughoutthe region。 but no one has ever found the source。 as john mcphee notes； it may be buriedunder glacially deposited soil； like the manson crater in iowa； or under the great lakes。
so how much do we know about what鈥檚 inside the earth？ very little。 scientists aregenerally agreed that the world beneath us is posed of four layers鈥攔ocky outer crust； amantle of hot； viscous rock； a liquid outer core； and a solid inner core。
1we know that thesurface is dominated by silicates； which are relatively light and not heavy enough to accountfor the planet鈥檚 overall density。 therefore there must be heavier stuff inside。 we know that togenerate our magnetic field somewhere in the interior there must be a concentrated belt ofmetallic elements in a liquid state。 that much is universally agreed upon。 almost everythingbeyond that鈥攈ow the layers interact； what causes them to behave in the way they do； whatthey will do at any time in the future鈥攊s a matter of at least some uncertainty； and generallyquite a lot of uncertainty。
even the one part of it we can see； the crust； is a matter of some fairly strident debate。
nearly all geology texts tell you that continental crust is three to six miles thick under theoceans； about twenty…five miles thick under the continents； and forty to sixty miles thickunder big mountain chains； but there are many puzzling variabilities within thesegeneralizations。 the crust beneath the sierra nevada mountains； for instance； is only aboutnineteen to twenty…five miles thick； and no one knows why。 by all the laws of geophysics thesierra nevadas should be sinking； as if into quicksand。 （some people think they may be。）1for those who crave a more detailed picture of the earths interior； here are the dimensions of the variouslayers； using average figures： from 0 to 40 km （25 mi） is the crust。 from 40 to 400 km （25 to 250 mi） is theupper mantle。 from 400 to 650 km （250 to 400 mi） is a transition zone between the upper and lower mantle。
from 650 to 2；700 km （400 to 1；700 mi） is the lower mantle。 from 2；700 to 2；890 km （1；700 to 1；900 mi） is the〃d〃 layer。 from 2；890 to 5；150 km （1；900 to 3；200 mi） is the outer core； and from 5；150 to 6；378 km （3；200 to3；967 mi） is the inner core。
how and when the earth got its crust are questions that divide geologists into two broadcamps鈥攖hose who think it happened abruptly early in the earth鈥檚 history and those who thinkit happened gradually and rather later。 strength of feeling runs deep on such matters。 richardarmstrong of yale proposed an early…burst theory in the 1960s； then spent the rest of hiscareer fighting