▁Mo ist ure ▁causes ▁concrete ▁to ▁collapse . ▁ ▁In ▁northern ▁cities , ▁g utt ed ▁sk ys c rap ers ▁and ▁buildings ▁are ▁filled ▁with ▁rain water ▁and ▁snow . ▁Rain ▁pour s ▁through ▁flo ors ▁and ▁ro ts ▁ce il ings . ▁The ▁changing ▁seasons ▁only ▁adds ▁pressure ▁to ▁the ▁buildings . ▁Rain water ▁see ps ▁into ▁cra cks ▁in ▁concrete ▁columns , ▁pill ars , ▁and ▁walls , ▁and ▁then ▁it ▁free zes ▁in ▁winter ▁before ▁th aw ing ▁again ▁in ▁spring . ▁These ▁repeated ▁free ze - th aw ▁cycles ▁split ▁and ▁crack ▁concrete . ▁Metal ▁bars ▁inside ▁concrete ▁gave ▁it ▁its ▁strength , ▁but ▁when ▁water ▁reaches ▁the ▁bars , ▁they ▁expand ▁and ▁crack ▁the ▁concrete . ▁Finally , ▁the ▁support ▁columns ▁give ▁way ▁and ▁the ▁buildings ▁to pp le . ▁The ▁upper ▁flo ors ▁rain ▁down , ▁sm ash ing ▁lower ▁flo ors ▁until ▁the ▁buildings ▁cr umble ▁to ▁the ▁ground . ▁Most ▁buildings ▁suffer ▁the ▁same ▁fate . ▁Sk ys c rap ers ▁around ▁the ▁world ▁begin ▁to ▁collapse . ▁Con crete ▁structures ▁such ▁as ▁train ▁stations ▁begin ▁to ▁fall ▁down ▁as ▁the ▁roof s ▁give ▁way ▁and ▁the ▁support ▁columns ▁break . ▁ ▁( 6 0 ▁years ▁A . H .) ▁Sea ▁life ▁has ▁completely ▁recovered ▁from ▁over f ishing ▁and ▁th riv es . ▁Though ▁there ▁are ▁still ▁dogs , ▁specific ▁dog ▁bre eds ▁do ▁not ▁exist ▁anymore , ▁er ased ▁by ▁gener ations ▁of ▁free ▁reprodu ction , ▁with ▁a ▁wide ▁variety ▁of