Fas­ci­nat­ing Facts About Un­seen Cities

Li­dar—a high-tech cousin of radar—uses light waves to peel back the lay­ers of time

Reader's Digest - - Contents - NI­COLA DAVIS FROM THE GUARDIAN

Li­dar—a cousin of radar—uses light waves to peel back the lay­ers of time.

WE USU­ALLY think of ar­chae­ol­ogy as in­volv­ing in­trepid ex­plor­ers and lots of painstak­ing dig­ging. But to­day, long-hid­den cities are be­ing re­vealed from the air, where mod­ern ar­chae­ol­o­gists use laser beams to spot ev­i­dence of an­cient life buried be­neath thick veg­e­ta­tion.

Li­dar, short for “light de­tec­tion and rang­ing” (and a cousin of ra­diobased radar), in­volves di­rect­ing a rapid suc­ces­sion of laser pulses— be­tween 100,000 and 400,000 per sec­ond—at the ground from an airplane or a drone. Soft­ware cap­tures the time and wave­length of the pulses re­flected from the sur­face and com­bines it with GPS and other data to pro­duce a pre­cise three­d­i­men­sional map of the land­scape be­low. These high-tech ex­plo­rations have re­vealed long-buried Mayan cities, in­clud­ing Tikal, in the dense jun­gle of Gu­atemala, and Cara­col, in Belize.

In re­cent years, li­dar ex­posed a sprawl­ing an­cient city in west­ern Mex­ico called Anga­muco. The dis­cov­ery of this long-lost Mex­i­can metropo­lis is es­pe­cially sig­nif­i­cant. Built by the Purépecha, who were ri­vals of the Aztecs, Anga­muco was a ma­jor civ­i­liza­tion in the early 16th cen­tury, be­fore Eu­ro­peans ar­rived.

“To think that this mas­sive city ex­isted in the heart­land of Mex­ico for all this time and no­body knew it was there is kind of amaz­ing,” says Chris Fisher, an ar­chae­ol­o­gist at Colorado State University who led the ex­pe­di­tion.

The city ex­tended over ten square miles be­fore it was cov­ered by a lava flow. “That is a huge area with a lot of peo­ple,” says Fisher. “You are talk­ing about 40,000 build­ing foun­da­tions, which is [about] the same num­ber of build­ing foun­da­tions that are on the is­land of Man­hat­tan.”

Ar­chae­ol­o­gists were sur­prised when they saw Anga­muco’s city plan. The Purépecha’s mon­u­ments—the city’s pyra­mids and plazas—were largely con­cen­trated in eight zones around the edges rather than be­ing lo­cated in one large cen­ter. Why this

al­most Los An­ge­les–type sprawl? His­to­ri­ans want to know the an­swer to that too.

The rev­e­la­tion of Anga­muco is a prime ex­am­ple of the power and prom­ise of li­dar. Ar­chae­ol­o­gists dis­cov­ered signs of the buried city in 2007 and ini­tially at­tempted to ex­plore it us­ing a tra­di­tional “boots on the ground” ap­proach. But the team soon re­al­ized that with the rugged ter­rain, it would take at least a decade to out­line the en­tire metropo­lis.

In 2011, they be­gan us­ing li­dar to map nearly 14 square miles, re­veal­ing an as­ton­ish­ing ar­ray of fea­tures, from pyra­mids and tem­ples to road sys­tems, gar­den ar­eas, and even ball courts. This gave them the

“map” they needed to know where to ex­plore fur­ther. So far, Fisher and his team have ver­i­fied more than 7,000 ar­chi­tec­tural fea­tures over a 1.5-square-mile area, with ex­ca­va­tions un­der­taken at seven lo­ca­tions. The ear­li­est ar­ti­facts in­clude ce­ramic frag­ments and other rem­nants dat­ing as far back as AD 900.

All told, re­searchers now be­lieve that more than 100,000 peo­ple lived in Anga­muco from about AD 1000 to AD 1350. That makes it the big­gest city in west­ern Mex­ico at the time— or at least the big­gest city we know about so far.

“Ev­ery­where you point the li­dar in­stru­ment, you find new stuff,” says Fisher. “Right now, ev­ery text­book has to be rewrit­ten, and two years from now, [they’re] go­ing to have to be rewrit­ten again.”

Li­dar re­vealed long-buried facets of Tikal, Gu­atemala, among other lost cities.

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