The world's treasures are under siege as never before

When Ernest Hemingway wrote "The Snows of Kilimanjaro," a holiday outing was the last thing he had in mind. Who could have known that this classic tale about a failed writer dying of gangrene in the shadow of Africa's tallest mountain would spark a stampede? Every year, some 10,000 vacationers huff their way to the 5,896-meter peak that untold tour operators have flogged with Hemingway's majestic words: "Wide as all the world, great, high, and unbelievably white in the sun." So it's poetic justice of sorts that the travel industry's purloined icon is melting. Thanks to global warming and deforestation, the millennial snowcap that was said to cover King Solomon's tomb is receding.

Those vanishing snows are emblematic of travel in a worrying new time—when no place can be taken for granted anymore. No matter how exotic the destination, until recently a traveler's biggest concern was how to get there, not where the journey would ultimately lead. Now thanks to rising incomes and falling airfares, getting there is the easy part; last year a record 806 million tourists hit the road. But those hordes—combined with forces ranging from climate change to civil war, industrial toxins to runaway development—are laying siege to some of the world's most treasured and irreplaceable sites.