The images flash by in a kind of inner slide show, and though, like most slide shows, they bring floods of happy memories, they also bring a certain wistfulness—and unease. I see the lonely, lamplit medieval lanes of Kathmandu as I first viewed them, barely 20 years ago, with few motorbikes in sight and the nearby mountains radiant. I see the temples of Angkor emerging from the jungle in which they had long been hidden. I see the Taj Mahal on the night I first visited—in 1974—when it was possible to enter after dark and to see the building by day without a brown shroud of smog. I even sometimes see the town of my birth, Oxford, when it seemed a living place and not a paved-over Universityland.

When I think of all that's being lost, sometimes in just a few years, I realize that some of it is in my head: Nowhere looks quite so startling as when first we met it, and for a certain kind of romantic traveler, every change is a change for the worse. And yet beneath all the tricks of the imagination, something else is true: Many of the wonders of the world are disappearing, because of human neglect or corruption or greed. The crucial question, then, is how to tend to these loved ones that are passing from our view.

The first thing we can do is to visit them, and the second—always helped by a visit—is to fight for their survival. In a global village in which the wonders of anywhere are, thanks to mass travel and mass communications, the glories of everywhere, we have to do what we can to protect the treasures of the planetary neighborhood, sometimes from ourselves.

I can still remember the first time I set foot in Lhasa, Tibet, in 1985, just after it had opened up to the world. A cluster of whitewashed traditional houses, bright with awnings and flower boxes, sat under the monumental gaze of the many-windowed Potala Palace, and life around the Barkhor marketplace was proceeding as it might have five centuries before. By the time I went back, just six years later, martial law had turned the city into a kind of open-air prison, with armed soldiers patrolling the rooftops and surveillance cameras everywhere. The last time I returned—at the beginning of this century, for this magazine—Lhasa was effectively a Far Eastern Las Vegas, gaudy with blue-glass shopping centers and high-rises. The Potala Palace was barely visible from most spots, and the few traditional buildings still in evidence were referred to as "Old Town," as if they belonged in an artificially preserved theme park.

All of us have our own examples, inevitably, and often they begin with our own hometown (Santa Barbara, where I grew up, had never seen valet parking or urban shopping malls until ten years ago). But in every corner of the globe, places are crumbling before our eyes, and only our voices can begin to reverse the process. In Bagan, the heart-stopping site of thousands of temples—white and earth-red and gold—set across the empty plains of Myanmar, a military dictatorship is busily destroying ancient buildings by "restoring" them for tourists with modern materials, and in nonsensical styles. In Old Havana, which was as quiet and peelingly authentic as any Spanish colonial settlement in the world when I used to visit 20 years ago, an economically desperate government is tarting up everything to attract foreign dollars. In Kyoto, near where I now live, an ancient capital that has withstood 12 centuries of earthquakes and fires and wars—it was even spared American bombs during the Second World War—is now being paved over by developers, who are razing old wooden buildings to make way for multistory concrete apartment blocks.