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All the signs of life that we've discussed so far are due to paratively simple forms （the methane in the rumens of cows is generated by bacteria that homestead there）。 Had your spacecraft flown by the Earth a hundred million years ago； in the age of the dinosaurs when there were no humans and no technology； you would still have seen oxygen and ozone； they chlorophyll pigment； and far too much methane。 At present； though； your instruments are finding signs not just of life； but of high technology—something that couldn't possibly have been detected even a hundred years ago：
You are detecting a particular kind of radio wave emanating from Earth。 Radio waves don't necessarily signify life and intelligence Many natural processes generate them。 You've already found radio emissions from other； apparently uninhabited worlds—generated by electrons trapped in the strong magnetic fields of planets； by chaotic motions at the shock front that separates these magnetic fields from the interplanetary magnetic field； and by lightning。 （Radio 〃whistlers〃 usually sweep from high notes to low； and then begin again。） Some of these radio emissions are continuous； some e in repetitive bursts； some last a few minutes and then disappear。
But this is different： A portion of the radio transmission from Earth is at just the frequencies where radio waves begin to leak out of the planet's ionosphere； the electrically charged region above the stratosphere that reflects and absorbs radio waves。 There is a constant central frequency for each transmission； added to which is a modulated signal （a plex sequence of ons and offs）。 No electrons in magnetic fields； no shock waves； no lightning discharges can generate something like this。 Intelligent life seems to be the only possible explanation。 Your conclusion that the radio transmission is due to technology on Earth holds no matter what the ons and offs mean： You don't have to decode the message to be sure it is a message。 （This signal is really； let us suppose； munications from the U。S。 Navy to its distant nuclear…armed submarines。）
So； as an alien explorer； you would know that at least one species on Earth has achieved radio technology。 Which one is it？ The beings that make methane？ Those that generate oxygen？ The ones whose pigment colors the landscape green？ Or somebody else； somebody more subtle； someone not otherwise detectable to a spacecraft plummeting by？ To search for this technological species； you might want to examine the Earth at finer and finer resolution—seeking； if not the beings themselves； at least their artifacts。
You look first with a modest telescope； so the finest detail you can resolve is one or two kilometers across。 You can make out no monumental architecture； no strange formations； no unnatural reworking of the landscape； no signs of life。 You see a dense atmosphere in motion。 The abundant water must evaporate and then rain back down。 Ancient impact craters； apparent on the Earth's nearby Moon； are almost wholly absent。 There must； then； be a set of processes whereby new land is created and then eroded away in much less time than the age of this world。 Running water is implicated。 As you look with finer and finer definition you find mountain ranges； river valleys； and many other indications that the planet is geologically active。 There are also odd places surrounded by vegetation； but which are themselves denuded of plants。 They look like discolored smudges on the landscape。
When you examine the Earth at about 100…meter resolution； everything changes。 The planet is revealed to be covered with straight lines； squares； rectangles； circles—sometimes huddling along river banks or nestling on the lower slopes of mountains； sometimes stretching over plains； but rarely in deserts or high mountains； and absolutely never in the oceans。 Their regularity； plexity； and distribution would be hard to explain except by life and intelligence； although a deeper understanding of function and purpose might be elusive。 Perhaps you would conclude only that the dominant life…forms have a simultaneous passion for territoriality and Euclidean geometry。 At this resolution you could not see them； much less know them。
Many of the devegetated smudges are revealed to have an underlying checkerboard geometry。 These are the planets cities。 Over much of the landscape； and not just in the cities； there is a profusion of straight lines； squares； rectangles； circles。 The dark smudges of the cities are revealed to be highly geometrized； with only a few patches of vegetation—themselves with highly regular boundaries—left intact。 There are occasional triangles； and in one city there is even a pentagon。
When you take pictures at a meter resolution or better； you find that the crisscrossing straight lines within the cities and the long straight lines that join them with other cities are filled with streamlined； multicolored beings a few meters in length； politely running one behind the other； in long； slow orderly procession。 They are very patient。 One stream of beings stops so another stream can continue at right angles。 Periodically； the favor is returned。 At night； they turn on two bright lights in front so they can see where they're going。 Some； a privileged few； go into little houses when their workday is done and retire for the night。 Most are homeless and sleep in the streets。
At last！ You've detected the source of all the technology。 the dominant life…forms on the planet。 The streets of the cities and the roadways of the countryside are evidently built for their benefit。 You might believe that you were really beginning to understand life on Earth。 And perhaps you'd be right。
If the resolution improved just a little further； you'd discover tiny parasites that occasionally enter and exit the dominant organisms。 They play some deeper role； though； because a stationary dominant organism will often start up again just after it's reinfected by a parasite； and stop again just before the parasite is expelled。 This is puzzling。 But no one said life on Earth would be easy to understand。
All the images you've taken so far are in reflected sunlight—that is； on the day side of the planet。 Something most interesting is revealed when you photograph the Earth at light： The planet is lit up。 The brightest region； near the Arctic Circle； is illuminated by the aurora borealis—generated not by life ； but by electrons and protons from the Sun； beamed down by the Earth's magnetic field。 Everything else you see is due to life。 The lights recognizably outline the same continents you can make out in daytime； and many correspond to cities you've already mapped。 The cities are concentrated near coastlines。 They tend to be sparser in continental interiors。 Perhaps the dominant organisms are desperate for seawater （or maybe oceangoing ships were once essential for merce and emigration）。
Some of the lights； though； are not due to cities。 In North Africa； the Middle East； and Siberia； for example； there are very bright lights in a paratively barren landscape due； it turns out； to burnoff in oil and natural gas wells。 In the Sea of Japan on the day you first look； there is a strange； triangular…shaped area of light。 In daytime it corresponds to open ocean。 This is no city。 What could it be？ It is in fact the Japanese squid fishing fleet； using brilliant illumination to attract schools of squid up to their deaths。 On other days； this pattern of light wanders all over the Pacific Ocean； seeking prey。 In effect； what you have discovered here is sushi。
It seems sobering to me that from space you can so readily detect some of the odds and ends of life on Earth—the gastrointestinal habits of ruminants； Japanese cuisine； the means of municating with nomadic submarines that carry death for 200 cities—while so much of our monumental architecture； our greatest engineering works； our efforts to care for one another； are almost wholly invisible。 It's a kind of parable。
BY THIS POINT your expedition to the Earth must be considered highly successful。 You've characterized the environment； you've detected life； you've found manifestations of intelligent beings； you may even have identified the dominant species； the one transfixed with geometry and rectilinearity。 Surely this planet is worth a longer and more detailed study。 That's why you've now inserted your spacecraft into orbit around the Earth。
Looking down on the planet； you uncover new puzzles。 All over the Earth； smokestacks are pouring carbon dioxide and toxic chemicals into the air。 So are the dominant beings who run on the roadways。 But carbon dioxide is a greenhouse gas。 As you watch； the amount of it in the atmosphere increases steadily； year after year。 The same is true of methane and other greenhouse gases。 If this keeps up； the temperature of the planet is going to increase。 Spectroscopically； you discover another class of molecules being injected into the air； the chlorofluorocarbons。 Not only are they greenhouse gases； but they are also devastatingly effective in destroying the protective ozone layer。
You look more closely at the center of the South American continent； which—as you know by now—is a vast rain forest。 Every night you see thousands of