修建中国长城的工程跨越了数千年。其中最著名的部分是明朝时期建造的蜿蜒的防御工事，建于14世纪至17世纪之间。它每年吸引着上千万的游客。与之相比，已有2000年历史的汉代长城——如今看起来有点像是堆放在沙漠中布满灰尘的饼干——吸引的游客要少得多。

Multi-annual measurements across Lake Superior indicate remarkable similarities between large lakes and ocean CO2 exchange during the ice-free season.

Click to print (Opens in new window) Click to email a link to a friend (Opens in new window) Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Click to ...

Click to print (Opens in new window) Click to email a link to a friend (Opens in new window) Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Click to ...