Inspired by the "Coulee verte" in Paris, the urban promenade snakes nearly 1.5 miles (2.4 kilometers) along Manhattan's West Side, providing stunning views of the skyline and Hudson River.
Stretching from Gansevoort Street in the trendy Meatpacking District to West 30th Street, it is set up on an old rail structure that was initially doomed for demolition.
But now the pedestrian-only park is replete with greenery that includes 300 kinds of carefully maintained perennials, grasses, shrubs and even trees.
Dutch landscape artist Piet Oudolf, who drew up the plans for the space, gave it a kind of wild feel that is in constant flux as the seasons change.
Locals and tourists alike flock to the High Line, open daily from 7:00 am to 11:00 pm in the summer, to relax or take in artistic installations or tours -- all for free.
A green oasis the city, it has become a popular spot to go for a walk, rest up, paint or simply soak in the scenery.
And it's proven to be quite a hit since the first section opened on June 8, 2009.
"This year, we expect over five million visitors, which is more visitors than the Statue of Liberty," said Jenny Gersten, executive director of "Friends of the High Line," the nonprofit that runs and maintains the park with an army of volunteers.
"It is quite extraordinary," she told AFP.
Gersten recalled how, when co-founders Joshua David and Robert Hammond first opened the High Line half a decade ago, "they didn't know what people were going to think."
"They opened the doors and said 'Will people come? How many people will come?'" she said. "And it has been a success beyond anything we imagined."
- New section opens soon -
A second section followed in June 2011 and construction is underway to complete a third, extending between 30th and 34th Streets and due to open by the end of the year.
"It is an experience of New York City that takes you up in the air, 30 feet (nine meters), to look out in ways that you can't have anywhere else," Gersten said.
The project took 10 years to materialize, starting with a neighborhood meeting in 1999 where co-founders David and Hammond pitched to preserve the weed-overgrown train tracks and turn it into a park.
"We only had a sliver of hope that it would happen," David, a former writer, told AFP.
Initial setbacks aside, the project slowly took shape. Former mayor Michael Bloomberg backed it, providing $112 million for construction that began in 2006.
That came on top of tens of millions of dollars raised by the "Friends of the High Line" group.
Today, 90 percent of the running costs and maintenance are covered by donations from individuals, foundations and corporations according to David, who calls the funding model "unusual."
Fifteen years since the start of his dream, he's still amazed at how far the project has come.
"It is such a magical project," David said, as High Line volunteers and friends enjoyed some birthday cake. "We thought we might have 400,000" visitors per year.
The High Line, which can get crowded on weekends, has in a matter of years transformed the neighborhoods it passes through.
Luxury buildings have sprung up alongside it, with others still under construction.
Marie Detree, visiting from Paris, said she was smitten with the park.
"I'm totally hooked," she said, admitting she had been returning each day.
"The design is very beautiful, very minimalist but functional at the same time ... it's amazing."