Aerial view of Lake Mead and land erosion near the border of Arizona and Nevada (© Lee Foster/Lonely Planet Images)

Aerial view of Lake Mead and land erosion near the border of Arizona and Nevada (© Lee Foster/Lonely Planet Images)

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Cleaning behavior of blue-striped grunt and four-eyed butterfly fish, Florida Keys National Marine Sanctuary -- Stephen Frink/Corbis

Cleaning behavior of blue-striped grunt and four-eyed butterfly fish, Florida Keys National Marine Sanctuary -- Stephen Frink/Corbis

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Toronto skyline during the Canadian National Exhibition, Ontario, Canada -- Gary Cralle/Getty Images

Toronto skyline during the Canadian National Exhibition, Ontario, Canada -- Gary Cralle/Getty Images

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Historical regatta in Venice, Italy -- SIME/eStock Photo

Historical regatta in Venice, Italy -- SIME/eStock Photo

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The pearl monument in the Corniche neighborhood of Doha, Qatar, illuminated at night with the new high-rises of West Bay in the background -- Jon Hicks/Corbis

The pearl monument in the Corniche neighborhood of Doha, Qatar, illuminated at night with the new high-rises of West Bay in the background -- Jon Hicks/Corbis

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Dead trees in Dead Vlei, Namib-Naukluft Park, Namibia -- Frank Krahmer/Corbis

Dead trees in Dead Vlei, Namib-Naukluft Park, Namibia -- Frank Krahmer/Corbis

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Aerial view of rice fields in Yunnan Province, China -- Jialiang Gao/Getty Images

Aerial view of rice fields in Yunnan Province, China -- Jialiang Gao/Getty Images

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