When you're sitting in the open-air amphitheater in the High Line park, overlooking Tenth Avenue — and the cars speeding by down below — it feels as if you are both one with the city and at the same time far away from the hustle and bustle of the streets beneath you. That is precisely part of the mission of Friends of the High Line, a nonprofit run by two neighborhood residents, Joshua David and Robert Hammond, since 1999 to preserve the elevated rail line from destruction and build a public park to transform the space from what once was a pickup and drop-off point for cattle-delivering trains into a garden in the sky.

A first phase of the High Line park, traversing the West Side of Manhattan from Gansevoort to 20th Street, opens to the public today, with a second section from 20th to 30th Street expected to open in 2010. To celebrate the occasion, David and Hammond, along with Anne Pasternak, president of the New York nonprofit Creative Time; James Corner, principal of James Corner Field Operations, the lead designer of the High Line; Ricardo Scofidio, principal of Diller Scofidio + Renfro, the High Line design team; Amanda Burden, chair of the City Planning Commission; Adrian Benepe, the parks commissioner; fashion designer Diane von Furstenberg and her husband, Barry Diller; and billionaire Philip Falcone and his wife, Lisa Maria Falcone, were in attendance at the High Line on Monday for a ribbon cutting and press conference.

The first section of the High Line includes a public art installation by Spencer Finch, whose work can be seen at the 53rd Venice Biennale this summer, and was commissioned by Creative Time. Stretching a full city block, from 15th to 16th streets, The River That Flows Both Ways was inspired by the Hudson River — one of the few rivers that flow in two directions. The work comprises 700 panes of glass, each meant to represent a moment on the river on a single day. To create the colors of the glass, no two of which are the same, Finch attached a camera to a boat on the Hudson and used a special instrument to photograph the river once a minute for 700 minutes, recording 700 different points in space and time.

Meredith Johnson, curator and producer of the consulting program at Creative Time, described being in the tugboat with Finch last summer for more than 11 hours. She says the beauty of the finished product, which is semi-enclosed in a tunnel, is that the artist was able to create a parallel between the movement of the railway and the river.

Delights like Finch’s installation can be found all along the High Line. Each block seems to have a varying ecosystem and design. There are shaded areas, differing types of flora and fauna, and diverse seating built into the walkway, including wooden lounge chairs for relaxing after gallery hopping in nearby Chelsea.

Some of the original rails were left in place to remind visitors of the site’s industrial past. James Corner, of the lead design team, said the design of the space is really about preserving the High Line’s sense of abandonment and melancholy, while creating a green ribbon in the sky. The question he says he tackled in the drawing room was, “How do you make it a public space and retain its same wild quality?”

You’ll have to go see for yourself.