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ll it water。
water is everywhere。 a potato is 80 percent water； a cow 74 percent； a bacterium 75percent。 a tomato； at 95 percent； is little but water。 even humans are 65 percent water；making us more liquid than solid by a margin of almost two to one。 water is strange stuff。 it isformless and transparent； and yet we long to be beside it。 it has no taste and yet we love thetaste of it。 we will travel great distances and pay small fortunes to see it in sunshine。 andeven though we know it is dangerous and drowns tens of thousands of people every year； wecan鈥檛 wait to frolic in it。
because water is so ubiquitous we tend to overlook what an extraordinary substance it is。
almost nothing about it can be used to make reliable predictions about the properties of otherliquids and vice versa。 if you knew nothing of water and based your assumptions on thebehavior of pounds most chemically akin to it鈥攈ydrogen selenide or hydrogen sulphidenotably鈥攜ou would expect it to boil at minus 135 degrees fahrenheit and to be a gas at roomtemperature。
most liquids when chilled contract by about 10 percent。 water does too； but only down to apoint。 once it is within whispering distance of freezing； it begins鈥攑erversely； beguilingly；extremely improbably鈥攖o expand。 by the time it is solid； it is almost a tenth morevoluminous than it was before。 because it expands； ice floats on water鈥斺�渁n utterly bizarreproperty；鈥潯ccording to john gribbin。 if it lacked this splendid waywardness； ice would sink；and lakes and oceans would freeze from the bottom up。 without surface ice to hold heat in；the water鈥檚 warmth would radiate away； leaving it even chillier and creating yet more ice。
soon even the oceans would freeze and almost certainly stay that way for a very long time；probably forever鈥攈ardly the conditions to nurture life。 thankfully for us； water seemsunaware of the rules of chemistry or laws of physics。
everyone knows that water鈥檚 chemical formula is h2o； which means that it consists of onelargish oxygen atom with two smaller hydrogen atoms attached to it。 the hydrogen atomscling fiercely to their oxygen host； but also make casual bonds with other water molecules。
the nature of a water molecule means that it engages in a kind of dance with other watermolecules； briefly pairing and then moving on； like the ever…changing partners in a quadrille；to use robert kunzig鈥檚 nice phrase。 a glass of water may not appear terribly lively； but everymolecule in it is changing partners billions of times a second。 that鈥檚 why water moleculesstick together to form bodies like puddles and lakes； but not so tightly that they can鈥檛 be easily separated as when； for instance； you dive into a pool of them。 at any given moment only 15percent of them are actually touching。
in one sense the bond is very strong鈥攊t is why water molecules can flow uphill whensiphoned and why water droplets on a car hood show such a singular determination to beadwith their partners。 it is also why water has surface tension。 the molecules at the surface areattracted more powerfully to the like molecules beneath and beside them than to the airmolecules above。 this creates a sort of membrane strong enough to support insects andskipping stones。 it is what gives the sting to a belly flop。
i hardly need point out that we would be lost without it。 deprived of water； the human bodyrapidly falls apart。 within days； the lips vanish 鈥渁s if amputated； the gums blacken； the nosewithers to half its length； and the skin so contracts around the eyes as to prevent blinking。鈥�
water is so vital to us that it is easy to overlook that all but the smallest fraction of the wateron earth is poisonous to us鈥攄eadly poisonous鈥攂ecause of the salts within it。
we need salt to live； but only in very small amounts； and seawater contains way more鈥攁bout seventy times more鈥攕alt than we can safely metabolize。 a typical liter of seawater willcontain only about 2。5 teaspoons of mon salt鈥攖he kind we sprinkle on food鈥攂ut muchlarger amounts of other elements； pounds； and other dissolved solids； which arecollectively known as salts。 the proportions of these salts and minerals in our tissues isuncannily similar to seawater鈥攚e sweat and cry seawater； as margulis and sagan have putit鈥攂ut curiously we cannot tolerate them as an input。 take a lot of salt into your body andyour metabolism very quickly goes into crisis。 from every cell； water molecules rush off likeso many volunteer firemen to try to dilute and carry off the sudden intake of salt。 this leavesthe cells dangerously short of the water they need to carry out their normal functions。 theybee； in a word； dehydrated。 in extreme situations； dehydration will lead to seizures；unconsciousness； and brain damage。 meanwhile； the overworked blood cells carry the salt tothe kidneys； which eventually bee overwhelmed and shut down。 without functioningkidneys you die。 that is why we don鈥檛 drink seawater。
there are 320 million cubic miles of water on earth and that is all we鈥檙e ever going to get。
the system is closed： practically speaking； nothing can be added or subtracted。 the water youdrink has been around doing its job since the earth was young。 by 3。8 billion years ago； theoceans had （at least more or less） achieved their present volumes。
the water realm is known as the hydrosphere and it is overwhelmingly oceanic。 ninety…seven percent of all the water on earth is in the seas； the greater part of it in the pacific； whichcovers half the planet and is bigger than all the landmasses put together。 altogether thepacific holds just over half of all the ocean water （51。6 percent to be precise）； the atlantic has23。6 percent and the indian ocean 21。2 percent； leaving just 3。6 percent to be accounted forby all the other seas。 the average depth of the ocean is 2。4 miles； with the pacific on averageabout a thousand feet deeper than the atlantic and indian oceans。 altogether 60 percent ofthe planet鈥檚 surface is ocean more than a mile deep。 as philip ball notes； we would better callour planet not earth but water。
of the 3 percent of earth鈥檚 water that is fresh； most exists as ice sheets。 only the tiniestamount鈥�0。036 percent鈥攊s found in lakes； rivers； and reservoirs； and an even smaller part鈥攋ust 0。001 percent鈥攅xists in clouds or as vapor。 nearly 90 percent of the planet鈥檚 ice is inantarctica； and most of the rest is in greenland。 go to the south pole and you will bestanding on nearly two miles of ice； at the north pole just fifteen feet of it。 antarctica alone has six million cubic miles of ice鈥攅nough to raise the oceans by a height of two hundred feetif it all melted。 but if all the water in the atmosphere fell as rain； evenly everywhere； theoceans would deepen by only an inch。
sea level； incidentally； is an almost entirely notional concept。 seas are not level at all。
tides； winds； the coriolis force； and other effects alter water levels considerably from oneocean to another and within oceans as well。 the pacific is about a foot and a half higher alongits western edge鈥攁 consequence of the centrifugal force created by the earth鈥檚 spin。 just aswhen you pull on a tub of water the water tends to flow toward the other end； as if reluctant toe with you； so the eastward spin of earth piles water up against the ocean鈥檚 westernmargins。
considering the age…old importance of the seas to us； it is striking how long it took theworld to take a scientific interest in them。 until well into the nineteenth century most of whatwas known about the oceans was based on what washed ashore or came up in fishing nets；and nearly all that was written was based more on anecdote and supposition than on physicalevidence。 in the 1830s； the british naturalist edward forbes surveyed ocean beds throughoutthe atlantic and mediterranean and declared that there was no life at all in the seas below2；000 feet。 it seemed a reasonable assumption。 there was no light at that depth； so no plantlife； and the pressures of water at such depths were known to be extreme。 so it came assomething of a surprise when； in 1860； one of the first transatlantic telegraph cables washauled up for repairs from more than two miles down； and it was found to be thicklyencrusted with corals； clams； and other living detritus。
the first really organized investigation of the seas didn鈥檛 e until 1872； when a jointexpedition between the british museum； the royal society； and the british government setforth from portsmouth on a former warship called hms challenger。 for three and a halfyears they sailed the world； sampling waters； netting fish； and hauling a dredge throughsediments。 it was evidently dreary work。 out of a plement of 240 scientists and crew； onein four jumped ship and eight more died or went mad鈥斺�渄riven to distraction by the mind…numbing routine of years of dredging鈥潯n the words of the historian samantha weinberg。 butthey sailed across almost 70；000 nautical miles of sea； collected over 4；700 new species ofmarine organisms； gathered enough information to create a fifty…volume report （which tooknineteen years to put together）； and gave the world the name of a new scientific discipline：
oceanography。 they also discovered； by means of depth measurem