This March 11, 2011 photo shows the Freedom Tower rising above the World Trade Center site and the square outlines of the 9/11 Memorial in New York. Rebirth has been slow in coming to ground zero, but with six months remaining until the first portions of the national 9/11 memorial are scheduled to open to the public, the work to turn a mountain of rubble into one of the world's great urban centers is thundering forward and beginning to produce some of the inspiring monuments designers envisioned nearly a decade ago. The square outlines of the 9/11 Memorial and the rising steel structure of 4WTC, top center, are among visible signs of progress at the site. (AP Photo/Mark Lennihan)

NEW YORK (AP) — The noise at ground zero is a steady roar. Engines hum. Cement mixers churn. Air horns blast. Cranes, including one that looks like a giant crab leg, soar and crawl over every corner of the 16-acre site.

For years, the future has been slow to appear at the site of the Sept. 11, 2001, attacks. But with six months remaining until the national 9/11 memorial opens, the work to turn a mountain of rubble into some of the inspiring moments envisioned nearly a decade ago is thundering forward.

One World Trade Center, otherwise known as the Freedom Tower, has joined the Manhattan skyline. Its steel frame, already clad in glass on lower floors, now stands 58 stories tall and is starting to inch above many of the skyscrapers that ring the site. A new floor is being added every week.

The mammoth black-granite fountains and reflecting pools that mark the footprints of the fallen twin towers are largely finished, and they are a spectacle. Workers have already begun testing the waterfalls that will ultimately cascade into a void in the center of each square pit. The plaza that surrounds them has the potential to be one of the city's awesome public spaces once construction is complete. Some 150 trees have already been planted in the plaza deck, even as workers continue to build it.

The memorial plaza won't be complete when it opens on Sept. 11, 2011, and a tour of the site last week makes clear that work around it will continue for years. Mud is still plentiful at ground level, and for now the site is dominated by the same concrete-gray shades that blanketed lower Manhattan after the 9/11 attacks.

But the agency that owned the trade center and has spent nearly a decade rebuilding it is aiming to deliver a memorial experience on 9/11/11 that closes one chapter — marked by mourning — and ushers in a new experience, where ground zero again becomes part of the city's everyday fabric.

"We want people to be able to see that downtown does have this incredible future to it," said Chris Ward, executive director of the Port Authority of New York and New Jersey. "The work will not be done on that day. What we hope will be done is the sense of frustration."

For now, the complexity and scale of the construction is evident in every corner.

Workers labor around the clock. During the busiest shifts, around 2,800 people — mostly men — labor amid tangles and ravines of steel. In one steel cavern that will become a transit hub concourse, showers of orange sparks fly as welders install trusses weighing up to 50 tons.

From the top of One World Trade, the view is spectacular, as it was from the twin towers, even though the building stands at 680 feet, less than halfway to its planned 1,776-foot height. Visitors to the upper floors can see the grand sweep of the Hudson River and New York Harbor, dotted with container ships, all the way to Sandy Hook at the northern tip of the Jersey Shore. People at ground level can now see the tower, too, from a growing number of places in the city and across the river in New Jersey.

High in the tower, safety is a big concern. There is netting everywhere to keep pieces of this or that from falling into the void below.

On the 29th floor, men preparing to install window glass last week were tethered to the building by safety cables as they worked near the ledge. Even their hard hats were attached by a safety line, in case they were knocked over the side. A yellow line, painted on the concrete deck, marked how close workers are allowed to stand without wearing a safety harness.

The building's glass curtain wall now rises to the 27th floor. After initial slow progress, the crews are getting better and faster at getting each pane in place, while managing wind that pushes each big sheet around like a sail. By Sept. 11, the building is expected to be 80 stories high, making it the tallest tower downtown.

A huge portion of the reconstruction of the trade center is taking place below ground. The underground halls that house the memorial are cavernous, and in their unfinished state look like some unexplored temple in an Indiana Jones movie.

The 60-foot-high slurry wall of reinforced concrete on the western edge of the site, meant to hold back the Hudson River, is two-thirds taller than Fenway Park's left field fence, and bears similarities in size and appearance to the Western Wall in Jerusalem.

The huge boxes that hold the waterfall pits visible from the surface are somehow suspended from the ceiling, held up by pillars that don't seem big enough to support the blocks' massive weight.

A maze of tunnels, catwalks and narrow, temporary staircases connect the various underground levels.

The complexity of the project is evident everywhere, but the choreography involved in keeping the place going is best demonstrated by the engineering feats that have been performed to prevent construction from disrupting the city's subway system.

The tunnel holding Manhattan's No. 1 subway tracks was buried beneath a mountain of rubble after the attacks. The tube now runs right through the middle of the site, hurdling thousands of passengers through ground zero every day.

To rebuild, work crews needed to excavate nearly 100 feet down below it, but rather than reroute service and demolish the tunnel, they merely propped it up on huge pilings and dug beneath it. The tracks, still encased in their old concrete tube, now sit suspended in mid-air as work takes place below, above and on either side.

Ward said he hoped people will be able to see in six months that, despite the ongoing construction, the site's days as a disaster zone are ending.

"It will be a place where you meet a friend for lunch. Where you meet a date. Where you race across the plaza and beneath the trees to get out of the rain," he said. "We want New Yorkers to make their own narrative there."