There is a new monument on the Mall. As big in feeling as in size, as brilliant in conception as in its startling red-orange color, it is the Hirshhorn Museum and Sculpture Garden's 25th birthday present to itself--an enormous work of art by American sculptor Mark di Suvero.

Actually, because it is not dedicated to any particular person, time or place, this may not be a monument in the technical sense. But it is prominent, beautiful and memorable. It lifts the heart and stays in the mind. It is a gift to the city and all who visit.

The artwork has been installed on a grassy plot right on the edge of the burrowed sculpture garden, just across Jefferson Drive SW from the museum building. This honored ground-level position gives the big piece extra visibility--it is the beacon the sculpture garden has ever lacked.

Like many pieces by di Suvero, this one, completed in 1967, has epic scale both literally and figuratively. Titled "Are Years What? (for Marianne Moore)," the piece is a dramatic sequence of nine red-painted steel I-beams welded and bolted together at sharp angles.

A central vertical beam anchors the whole--other beams pinwheel from it in varying lengths. The tallest of these streaks diagonally upward for about 40 feet. Suspended with heavy-duty cable from the tip of this diagonal is a beam that has been bent into a tight V-shape. This weighty form moves in the wind with improbable grace, further animating an already vivid ensemble.

The title refers to a short poem by Moore, the epigrammatic American poet who died in 1972. In three stanzas, Moore celebrates the ironies of life. "The very bird, grown taller as he sings, steels his form straight up," she wrote. "Though he is captive, his mighty singing says, satisfaction is a lowly thing, how pure a thing is joy. This is mortality. This is eternity."

Connections between the words and the sculpture are of course only indirect, but both are earthbound and soaring. There is a bit of magic in both, too. Di Suvero takes materials intended for other purposes--steel beams made to support bridges or skyscrapers--and transforms them into objects of desire and aspiration.

Di Suvero, 66, is one of the world's great living artists. He was born in Shanghai to Italian parents who, in a romantic (and possibly homesick) gesture, named him Marco Polo di Suvero to commemorate Venetian roots the family shared with the fabled explorer.

The family moved to San Francisco in 1940. As an adventurous young man di Suvero once sailed the entire California coast alone in a skiff. He also found the time to acquire a degree in philosophy from the University of California at Berkeley.

Besides studying and sailing di Suvero began fatefully to make art. By the late '50s he was living in New York and forming sculptures out of huge pieces of discarded wood and steel and stuff such as rubber tires or old ladders that he found around the city.

The sculptures were forceful and dynamic--they took the vitality found in the best gestural paintings of the abstract expressionists and moved it into three dimensions. It was as if a Franz Kline painting, say, had been suddenly and astonishingly exploded.

These early sculptures were big, but they still fit into rooms. By 1967 even loftlike enclosures no longer were large enough to hold the steel sculptures he was fabricating. Di Suvero loved the hard labor, and it shows in the gutsy slices, welds and bends in pieces such as "Are Years What? (for Marianne Moore)." Di Suvero is the John Henry of sculptors--his pieces are celebratory, and among the things they celebrate is sweaty, gritty work.

Made for the outdoors, the sculptures look equally at home in cities or in open fields. Partly this is due to sheer size--the pieces have the heft to be seen from a distance and to stand up to other mammoth objects, be they trees or buildings.

But it also is a matter of contrast and harmony. In materials and color, "Are Years What?" asserts its man-made identity without compromise. It would stand out in any setting. Yet its forms are not antagonistic; somehow they harmonize with both rural and urban surroundings. Despite the hard edges and sharp angles, the work has an organic spirit. Among other things, you can think of it as a growing tree or as a dancing human.

Like many di Suvero sculptures, "Are Years What?" has led a peripatetic existence. In the 32 years before finding a permanent home on the Mall it appeared in a dozen different locations, including the rolling countryside of the Storm King Art Center in Upstate New York and the Esplanade des Invalides in Paris. Hirshhorn Director James Demetrion first encountered the work 23 years ago in Brooklyn's Prospect Park, where it was installed as part of a di Suvero retrospective that placed pieces in all five New York City boroughs.

For Demetrion, as for many others, it was love at first sight, and he doggedly waited for his chance--the piece was acquired early this year with private funds and in exchange for "ISIS," a 1978 di Suvero piece the Hirshhorn owned. "ISIS" is a wonderful piece but, with its suspended ship's prow, it lacks the airy, exquisite balances of the new acquisition. Installed for more than a decade on the plaza next to the cylindrical Hirshhorn building, "ISIS" was shipped out on loan to a Chicago-area sculpture park eight years ago when the plaza's renovation began.

"Are Years What?" is an excellent reminder of how swell it has been to have the Hirshhorn here for a quarter of a century, in particular the splendid sculpture garden. Joseph Hirshhorn changed the cultural landscape of the city when he came along with his fabulous gift of art, and it was he who rightfully insisted that "Sculpture Garden" be an integral part of the museum's name.

The garden wasn't so gardenlike in its initial incarnation--architect Gordon Bunshaft wanted a spare, Zenlike atmosphere for Hirshhorn's sculpture, and the result was a rather barren landscape, sizzling hot in summer. Seven years later a successful greening took place. Under the guidance of landscape architect Lester Collins, arid expanses were replaced with grass, vines, low-lying plants and trees.

But cost-cutting affected both the first and second garden designs, with unfortunate results that remain visible to this day. All that remains of the restaurant and open tunnel Bunshaft envisioned is an ugly garage-like door for staff use only. The plain pipe-rack handrails of Collins's renovation continue to mar the ramps and stairwells.

And then there is the black hole for trees. Bunshaft carefully placed a weeping willow on a low mound next to the little reflecting pool--a beautiful accent at a critical place. But the tree died, and so have all of its replacements. Such (relatively) little things mean a lot.

The garden was never piped for irrigation or drainage, explains Nancy Bechtol, chief of the Smithsonian's Horticulture Services Division. She's getting at the problem little by little, but it is costly. It seems clear that the Hirshhorn needs to pay closer attention to the upkeep of its garden--an endowment for its maintenance would be a good idea.

Still, the Hirshhorn has the big things right--Auguste Rodin's "Balzac" and "Burghers of Calais," Aristide Maillol's "Action in Chains," David Smith's "Pittsburgh Landscape" and "Voltri XV," and many, many other fine works of art. The list now proudly and fittingly includes Mark di Suvero's magnificent "Are Years What? (for Marianne Moore)."