It seems improbable, but it's one of the new glories of New York that all any of us has to do to radically reshape our thoughts about the city is to climb up a wide flight of steps. Or, even easier, take a quick elevator ride.

The steps and elevators are the entrances to the High Line, New York's first great 21st-century park, a lushly planted, all-upstairs affair that—unlike any other park in the city—follows the course of an abandoned elevated freight train line a mile and a half long and 30 feet high; it's a viaduct turned strolling ground that 80 years ago got tucked in alongside old warehouses and factories and tenements on the far west side of lower midtown Manhattan (only a short walk south of where the Javits Center now stands, and thus easy to get to on even a short BEA break). It was forgotten by almost everyone until, it seems, just the other day.

On arriving at the top, a ribbon of paths and trees and grasses stretches before you; from there, what is invisible from the ground is clearly seen—how the random pieces of Manhattan fit together in a way that keeps it going day after day. Beyond that, and even more surprisingly, you take in an exhilarating, lingering, bittersweet sense of the long sweep of New York life across time—the kind of vision that inspired Melville, Whitman, and other writers amazed by the city.

A further improbability: from outside and beneath, the park's container and wrapper—a narrow black steel sluiceway on stilts—looks so demure and austere that the High Line itself barely seems to exist until you've climbed aboard. Yet less than a year after opening ceremonies in June 2009, more than two million people have already found their way to and up into this beautiful park.

But perhaps improbability is both the bedrock and building block of reality. Certainly several long lines of unlikely events had to converge and intertwine before the High Line could emerge as capable of having such a transforming effect on the people who flock to it. The origins of the new park trace back, oddly enough, to a whim of Commodore Cornelius Vanderbilt, one of the original robber barons, who after the Civil War, when he had made himself master of New York's railroads, decreed that from then on nearly all Manhattan passenger trains would run down the east side of the island, while freight trains would be shunted over to the west side. Over time, this decision endowed the east side with the great palace of Grand Central Terminal and the proud towers of Park Avenue (a street built on top of covered, sunken tracks); the west side initially had to make do with tracks through the streets, a calamity finally remedied decades later by construction of the High Line. When freight trains stopped running through Manhattan in 1980, the area was left with only a relic and a patchwork of abandoned rail yards.

Two successive upwellings of farsighted and imaginative stubbornness brought about the High Line's transformation from an eyesore to a sight for sore eyes. Just when the demolition of the whole structure appeared imminent, the effort was thwarted thanks to three West Side activists who could see beyond the obvious. First, Peter Obletz, a real estate and transportation consultant whose dream was to bring trains back to the High Line, bought the moldering rail corridor in 1984 for $10. Obletz then spent the rest of his life fighting developers and the city, which in court papers called the line “an anachronistic blot on the landscape.” After his death in 1996, two others stepped forward—Joshua David, a writer, and Robert Hammond, an artist and entrepreneur. Their idea was to make the train corridor a place for people; in 1999 they cofounded Friends of the High Line and raised enough money (more than $130 million) to design and build the park.

The park—boldly, uniquely—celebrates what David and Hammond saw the first time they clambered up onto the derelict rail line Obletz had kept standing: a rare moment of equilibrium between two of the most powerful forces in New York's history—relentless development and a resilient natural landscape. As soon as human maintenance of the tracks was withdrawn 30 years ago, natural reclamation began; by the time David and Hammond reached it, the High Line had become a self-seeded semiwilderness of small trees and tall grasses that might make Whitman swoon. Traditionally, or at least ever since the opening of Central Park in the 1850s, New Yorkers have placed high value on both buildings and on the open spaces left unbuilt. But until now they have frowned on seeing the two too closely mingled—even though every crack in the sidewalk brings forth a few blades of green struggling for a foothold.

The new High Line, on the other hand, welcomes these juxtapositions. The result is a park that seems almost to have been cocreated by natural processes, by old Commodore Vanderbilt, and by his great contemporary, Frederick Law Olmsted, the designer of Central Park: 210 species of carefully replanted perennials, grasses, and trees intertwine in striking and never-before-seen shapes and patterns both with carefully restored lengths of track and with a brand new system of elegant, twisting concrete paths that widen and narrow at irregular intervals.

Just as unusual is the quiet of the park—you're right in the middle of the city and yet, perhaps because of the park's height, perhaps because the uplifted landscape feels surprisingly spacious and enveloping once you enter it, or perhaps because the High Line follows a mostly midblock passage through the resurgent meat-packing district and next-door Chelsea, the sounds of traffic from the avenues the park parallels and the streets it crosses are muffled or somehow held at bay. What you sense far more strongly are breezes off the Hudson River a short distance to the west and clouds crossing the wide-open sky overhead (the park has already become a favorite with sunset watchers).

Then there are the park's unprecedented views of New York itself: there is something about hovering 30 feet above the city as it goes about its business that lends just the right sort of distance for seeing things clearly, since you can look around for miles and at the same time still feel fully engaged in all the activities that can be glimpsed below. You can gaze at the Empire State Building or peer into second-floor apartments or inspect a hot dog vendor or kebab stall's action from above. Looking at nearby buildings you can see some of the thought and hope that went into putting them together, even if from the street some of the same structures look more like haphazard piles of brick and steel. What stands out particularly from on high are qualities that are often hidden when walking at street level—a purposefulness and a kind of orderly grace in the confluence of pedestrian and vehicle.

Access to thoughts like these is perhaps the most unsuspected of the High Line's accomplishments. But remarkable locations and unexpected sights and sounds can shift our minds into a different kind of awareness, a vivid, memorable, highly energized state with its own startling qualities—prompting us to take, for instance, a suddenly deepened interest in everything around us. I call this wide-awake state—which is built into all of us but normally remains as inconspicuous as the High Line does from the street—Deep Travel. To me, Deep Travel is the great reward and the “second destination” of all travel, whether we're far from home or just moving from one neighborhood to another.

Deep Travel has a further characteristic, too: it can alter our perceptions of how time passes, greatly extending the reach of the present moment, so that we sense an unbroken connection to some still ongoing event or place as it unfolds. Sometimes this sense of the long span is triggered by specific sites or spots—crossing from Brooklyn on a ferry, Whitman found himself in the presence of New Yorkers from future generations:

Just as you are refreshed by the gladness of the river and the bright flow, I was refreshed;

Just as you stand and lean on the rail, yet hurry with the swift current, I stood, yet was hurried…

What is it, then, between us?

What is the count of the scores or hundreds of years between us?

Whatever it is, it avails not—distance avails not, and place avails not.

For Melville, at the beginning of Moby-Dick, it was the shoreline of Manhattan that induced Deep Travel thoughts: “Circumambulate the city of a dreamy Sabbath afternoon,” he wrote. “What do you see?—Posted like silent sentinels all around the town, stand thousands upon thousands of mortal men fixed in ocean reveries.”

Up on the High Line, and especially in a small corner of the park called 10th Avenue Square, the almost 400 years of New York's tumultuous history as a permanent settlement are on open display in the center of town. This is the one place along its length where the High Line crosses a major avenue, and to bring you even closer to this endless, restless flow of humanity, there's a small amphitheater and overlook. Wooden benches dip down toward the street, and department store–sized plate-glass windows have been cut into the giant support beams that hold the park aloft. This puts you about at the height of some imaginary triple-decker bus, very close to the pulse of traffic surging just below and yet as immune from it as if you were lounging in a Venetian street or cafe. It's a magical spot, where you can safely dip your toe into New York's swift current.