MILL RUN -- The waters are still falling under the apricot-colored cantilevered terraces in a soothing rush of sound, and every day -- except Wednesdays -- visitors, hushed and gap-jawed, trudge through the great house, up and down the narrow flagstone staircases, gazing at the vistas, the Picassos and the Diego Riveras, the Cherokee red cast iron kettle, the rocks erupting out the floor by the fireplace.

Fallingwater's 75th anniversary is this year, and the Western Pennsylvania Conservancy has made sure to celebrate Frank Lloyd Wright's architectural masterpiece with parties, publication of a sumptuous coffee-table book by Rizzoli International Publications, a documentary by Pittsburgh filmmaker Kenneth Love with restored interviews of the late Edgar Kaufmann Jr. and an exhibit of plein air paintings by Felix de la Concha.

In addition, there are efforts under way to get Fallingwater designated as a World Heritage Site -- a first for this region.

These days, the house's interior gleams like a burnished jewel, despite some leaky skylights. It has inspired a cookbook, a Lego toy set with 810 pieces and a line of paint colors. There was even a Twitter chat this summer about the house -- built between 1936 and 1939 by Pittsburgh's Kaufmann department store family -- sponsored by the American Institute of Architects, whose members in 1991 declared Fallingwater "the best all-time work of American architecture." .

"I heard someone once refer to Fallingwater as Frank Lloyd Wright singing, and it's true," says Lynda Waggoner, the conservancy's vice president and longtime director of Fallingwater, sitting in her tiny, cozy office just off one of those narrow stairways on a recent damp autumn day.

Instead of building a nice house overlooking the falls -- Bear Run, which tumbles down to the Youghiogheny River a third of a mile away -- Wright made water "a part of the house," she noted. In the front entryway, water piped in from the stream even trickles through the stone. "It's such a full-blown example of his genius," she said. "There's no better expression of organic architecture -- and for a weekend house, it served the Kaufmann family very well."

In July, Secretary of the Interior Ken Salazar announced that Fallingwater and 10 other Frank Lloyd Wright houses -- his portfolio included about 400 buildings in all -- would be nominated for inclusion on the United Nations' World Heritage list, which already includes the Grand Canyon, Independence Hall in Philadelphia and the Statue of Liberty, not to mention the Taj Mahal and the Pyramids of Giza.

The formal nomination by the U.S. won't happen until 2013. And while becoming a World Heritage site doesn't necessarily mean more money, it's an honor, Ms. Waggoner said, "like winning an Academy Award."

It's been a good run for Ms. Waggoner, who has been with the conservancy off and on since 1965 -- when she, then a high schooler, showed up at the site looking for a summer job, two years after the Kaufmann family gave the house and 1,600 acres to the conservancy. In 1985, after 10 years working elsewhere after college, she was hired as Fallingwater's first curator before being named director in 1996.

During those years, the annual tally of visitors has risen to 160,000 annually. Its collections have been cataloged and conserved, the building restored and its foundations strengthened after an engineering student discovered that the building was actually in danger of collapse.

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Through the years, the Kaufmanns, who were creative, worldly, rich -- dubbed "the merchant prince and princess of Pittsburgh" by Time magazine in 1938 -- received many famous guests at Fallingwater, including Albert Einstein, Isaac Stern and Wright himself, while later visitors have included Hillary Rodham Clinton, Brad Pitt and Steve Jobs. Today, Fallingwater is surrounded by 5,000 protected acres of pristine woodland, Ms. Waggoner said. "We are well protected and in great shape for the future."

While no one disputes its singular importance, David Netto, an interior designer writing in The Wall Street Journal in May, argued that Fallingwater is not truly a modern house, not having launched a formal school of architecture or thousands of less expensive imitators.

Fallingwater, he wrote, "has often been referred to as a masterpiece with no progeny" because its construction was completed in the 1930s "when the 'industrial' strain of modernism would finally conquer the handmade."

Other architects virulently disagree.

Fallingwater "is a modern modernist modernistic building born of modernity," countered architectural historian Barbara Lamprecht. Like other modernist masterpieces, it "fully exploits 20th-century issues of freshly conceived spatial relationships; a newly kinetic interaction among outdoors, indoors and human being; the radical importance of the diagonal view in which movement is implicit."

Ms. Lamprecht ticked off numerous reasons that Fallingwater is so very modern: the design's asymmetry, the use of the same materials inside and outside, "the alternating unornamented rectilinearity of powerful solids punctured and balanced by equally powerful voids and a rhythm as self-confident and as apparently indifferent to blending in with 'nature' as any house by [Walter] Gropius, Le Corbusier or Richard Neutra." She notes that a lot of modernist houses "didn't have progeny ... and Fallingwater is so site-specific."

If modernism, as art critic Robert Hughes wrote, is the "shock of the new," Mr. Love, the filmmaker, said that in the year he spent shooting his latest documentary about the house, he'd always be startled when rounding a corner.

"Every morning I'd come around the bend and there'd be this 'aha' moment, but every day was different," he said. "Fallingwater is so consistently fresh."

Mr. Love's 1994 film, "Fallingwater: A Conversation with Edgar Kaufmann Jr." was itself refreshed and restored so that its latest widescreen and high-definition iteration not only includes Kaufmann home movies from 1936 but also the younger Kaufmann's interviews for an early documentary. Mr. Love served as the sound man on that project and still marvels at the eloquence and clarity of Kaufmann, who died in 1989.

Mr. Love's new documentary, titled "Fallingwater: Frank Lloyd Wright's Masterwork," was mostly shot with a Canon 5D camera on Wednesdays when the site was closed to the public. It was recently screened at the Three Rivers Film Festival and was also shown at the Guggenheim Museum's First Fridays Series in New York.

"I'm getting calls from France, Italy and Japan by people who want to see it," he said, adding that he's also hoping that proceeds from the sale of this film will pay for the creation of a 3-D version that will serve as a virtual tour for disabled Fallingwater fans. (Because of its many stairs and narrow doorways, the home is not accessible to people with disabilities.)

"As I mature, my understanding of the house has completely changed. I see so much more clearly the years of craftsmanship, the grains of the wood, the organic connection of the building with the landscape. It's like walking into a picture frame of art, where suddenly all the dimensions appear around you. I've taken Japanese friends to Fallingwater, and they understand it immediately. Their understanding is intuitive and instantaneous."

Americans, too, responded positively once the first stories about the house began to appear in newspapers and magazines.

Despite being built in the depths of the Depression, "the woodland retreat over the waterfall had a fast track into the American psyche," wrote Robert Gangewere in Carnegie Magazine in 1999, noting that it was "produced at a time when Hollywood was creating escapist fantasies of its own about avoiding economic hardship. Millions of Americans, including unemployed workers in Western Pennsylvania, could dream about life in a private retreat created by the most famous architect in America."

While books and films do the house justice, nothing replaces the experience of being there, said Mr. Love. Kaufmann Jr. insisted that no velvet ropes be installed and, in a memo quoted in the Rizzoli book, instructed that the house's collections be treated "decently and listed adequately," but always "in full subordination to the real values present."

Today, an hour's tour -- our guide was the cheerful Henry Moore ("this is what I do when I'm not sculpting," he joked) -- feels like a walk through a home, not a museum; Kaufmann Jr. told guides not to be too scholarly but friendly and approachable. While Mr. Moore gives the obligatory warnings not to take pictures indoors or lean against the master terrace's wall, his love for Fallingwater is palpable.

"It shows me something new on every tour," he says.

While Fallingwater is the work of a mature genius who spent years perfecting the project, the architect famously dashed off the first drawings in just a few hours, after Kaufmann Sr. unexpectedly called to say he was en route to Wright's Wisconsin studio, Taliesin, to check on what he had commissioned a year earlier.

Lest you think the house was designed to be perfect, think again: In her introduction in the big Rizzoli book, Ms. Waggoner recounts how she discovered an important Tiffany vase had been broken and awkwardly repaired and went to Kaufmann Jr. to apologize about what she thought might have been clumsy housekeeping.

"He smiled at me and said, 'Lynda, don't blame the staff. I broke that vase, and I glued it back together. You don't understand. Fallingwater was our weekend house. This is where all the things that were damaged but too good to throw away ended up.' "

For all his casualness, the younger Kaufmann never doubted the structural integrity of the house, but Wright was no engineer. His houses were notoriously leaky, but this house was deeply structurally flawed, with problems foreseen even during its three-year construction when the elder Kaufmann's own concrete engineers said more steel was needed to support the second floor.

After University of Virginia engineering student John Paul Huguley discovered slippage in Fallingwater's foundation in 1995 -- ironically in a project meant to show the brilliance of the design -- experts were called in. Robert Silman, an internationally renowned architectural preservationist, monitored the building's movements for 17 months and realized that the building's famed cantilevers -- the single defining architectural feature of the house supporting the seemingly weightless terraces and glass walls -- were failing.

Wright's gargantuan ego made him acutely sensitive to criticism. And while the two men were strong collaborators and friends, when the elder Kaufmann wrote to him in 1936 questioning whether the concrete and steel supports were sufficient, he responded with fury:

"My dear E.J," Mr. Wright wrote, in a letter preserved in Western Pennsylvania Conservancy archives. "If you are paying to have the concrete engineering done down there, there is no use whatever in our doing it here. I am willing you should take it over but I am not willing to be insulted. ... I don't know what kind of architect you are familiar with, but it apparently isn't the kind I think I am. You seem not to know how to treat a decent one. I have put so much more into this house than you or any other client has a right to expect that if I haven't your confidence -- to hell with the whole thing.

"Sincerely yours, Frank Lloyd Wright, Architect."

Kaufmann didn't build a department store empire without learning something about people, and he knew exactly how to handle the brilliant, if cantankerous, architect.

"Dear Mr. Wright: If you have been paid to do the concrete engineering up there, there is no use whatever of our doing it down here. I am not willing to take it over as you suggest, nor am I willing to be insulted. ... I don't know what kind of clients you are familiar with, but apparently they are not the kind I think I am. You seem not to know how to treat a decent one. I have put so much confidence and enthusiasm behind this whole project in my limited way, to help the fulfillment of your efforts that if I do not have your confidence in the matter -- to hell with the whole thing.

"Sincerely yours, Edgar J. Kaufmann.

"P.S. Now don't you think that we should stop writing letters and that you owe it to the situation to come to Pittsburgh and clear it up by getting the facts?"

Obviously, they did clear it up, and 75 years later, Pittsburghers and lovers of architecture from all over the world are the better for it.

First Published: November 27, 2011, 10:00 a.m.