THE TREASURY DEPARTMENT, legend says, was built just where it is at 1500 Pennsylvania Avenue, between 15th Street and East Executive Avenue, so the Congress couldn't see what President Andrew Jackson was up to.

Some say the President chose the site to keep the nation's money at his right hand. True, a tunnel runs between the White House and the Treasury Department. Another tunnel goes under Pennsylvania Avenue over to the Treasury Annex.

Actually, President Franklin Delano Roosevelt constructed the tunnel as an air raid shelter at the sub-basement level during World War II. The official word is that the tunnel was boarded up many years ago. But if so, how did Tricia Nixon after her wedding get out of the White House without being seen by the waiting press?

When we went through the building with the Columbia Historical Society the other day, we didn't see the tunnels, but we did walk through the vaulted halls, the Cash Room and the 1864 burglar-proof vault as well as the suite of rooms that once served Andrew Johnson as a temporary White House.

Unless you walk around the Treasury every day, the proper thing to do is get there early enough to walk all around the building first. In not too many weeks, the Treasury's glorious magnolias bloom, sounding grace notes against the edifice's Dix Island, Maine granite. The original east wing, Acquia Creek freestone, came from George Washington's own quarry. But the stone crumbled, and the granite replaced it in 1908.

The neoclassical east front, built between 1836 and 1869, marches down 15th Street in an impressive Ionic colonnade: 30 columns, each 36 feet tall and carved out of a single block of granite.

Robert Mills designed a T-shape: the colonnade and a center stem wing. He was also architect of the Washington Monument and the Patent Office Building (now the National Portrait Gallery).

Mills got the job for his expertise in fire-proof buildings, (using groined arches, cemented and coated with hydraulic cement) after two previous Treasury buildings were burned down, by the British in 1814 and by an arsonist in 1833.

Thomas Ustick Walter, famous for his Capitol dome, designed the south wing, (1855-1860) as well as the west (1855-1864). With these extensions, the view of the White House from the Capitol -- and vice versa -- was fully cut off.

The south portico and plaza dominate Treasury Place, guarded by Alexander Hamilton, the first Treasury secretary. James Earle Fraser sculpted the bronze statue in 1923 for a base by Henry Bacon. James Goode in The Outdoor Sculpture of Washington, D.C. claims an anonymous veiled lady paid for it.

Ammi B. Young and Isaiah Rogers supervised construction on the south and west wings. Their interior frills and thrills reflect ornament in the mid-19th-century fashion: iron columns reinforcing Mills' brick vaults and the elaborate leaf and fascia decoration on the steps that spiral through the building.

On the north plaza of the Treasury, as you pass the statue of Thomas Jefferson's Treasury Secretary Albert Gallatin (1761-1849), tip your hat and tell him how we need him now. Gallatin raised the money to pay off the deficit left by Hamilton and even produced the first Treasury surplus. In the 1930s, Democrats raised private money for Gallatin's statue. The Republican congress wouldn't vote it, not wanting to put up a rival to their man, Republican Hamilton. James Earle Fraser sculpted the bronze statue, erected in 1947, at Pennsylvania Avenue and 15th Street NW.

Alfred B. Mullett constructed (1867-1869) the impressive north wing, with its Pennsylvania Avenue portico and the great Cash Room. At completion, the five wings formed two interior courtyards. Mullet also designed the tiered wedding cake: the State, War and Navy Building (now called the Old Executive Office Building).

CASHING IN ON THE TOUR

We arrived at the wrong door on the east front of the Treasury (the 15th Street side) dutifully five minutes before the 10 a.m. tour. (To find the right door, ask when making reservations.)

The only public entrance is on the lower floor, below the colonnade, on the east, a mean and dingy substitute for Mills' original ceremonial entry up steps and through the colonnades.

Be sure to bring good identification -- security is stiff here. Leave your keys and metal jewelry at home or the super sensitive metal detector will whistle at you as you go through.

Our tour took about an hour, under the guidance of Katherine Whitney. Chief Curator Judith Lanius explained that what we'd see is a work in progress -- the grandest and most historic rooms under restoration, though much is already done.

We climbed what seemed a great many Young/Rogers steps, holding tightly onto the shining brass railing, regretting heavy coats, rejoicing in flat Rockport shoes. The vaulted halls, the first areas to be restored, are a lovely lofty view. With the Columbia Historical Society Tour, whose members often amplified the guide's lecture, we walked on the original marble-square floors. If you look carefully, as Whitney pointed out, you can see lighter-colored marble which replaced the original glass blocks, serving as skylights from top to bottom. The original oculi, round windows in the domes, eventually will be freed from their later concrete lids and will once again light the stairwells.

Portraits of past Treasury secretaries guard the corridors. There's George Shultz, looking more rested than he does as secretary of state. We took an illegal look -- it isn't finished yet -- at the suite being prepared as an entertaining/conference room for Secretary James A. Baker: ornate (eagle and fruit) marble fireplace, gilded mirror, ceiling decorated with pastel circles and flowers, and eventually, period furniture.

President Andrew Johnson ran the nation from a suite on the west wing during the two months it took to get Mary Todd Lincoln out of the White House after the death of her husband. In one of the old engravings on the wall, you can see the same sofa there now.

In the treasurer of the United States' office, we admired the decorative cast-iron wall, the only part left of four original burglar-proof vaults built in the northwest corner of the Treasury in 1864.

The vaults' inventor, Isaiah Rogers, devised a lining of two layers of cast iron balls, held in cavities between wrought iron and hardened steel plates. If some miscreant drilled the wall, the balls just rolled.

Fortunately, in 1985 when the Treasury redecorated the room, construction people found the vault wall, hidden for 80 years under layers of newer walls and restored it. Some thoughtful person also left a section of wall exposed so we saw just how the balls work.

Last stop is the north wing, designed by Mullett. Its prime room is the Cash Room, a balconied marble hall that's 72 by 32 feet and 27 1/2 feet high. In 1869, President Ulysses S. Grant and the room were both inaugurated at a ball. According to Whitney, it was an awful party -- some people didn't get their coats back until the next day. And the food servers couldn't get the vittles to the table through the crush.

But the room itself shines with nine kinds of marble and bronze. One of the historical society members said he'd often cashed checks at the long cash counter (now gone). As late as 1975, government workers could cash their checks, and the public could buy Treasury notes there.

We left eager to see the next stage of the restoration.

TREASURY TOURS --

Alternate Saturday mornings. You must make reservations by calling the Curatorial and Preservation Office at 535-9830. If you have so much fun you want to do it again and again, volunteer to be a docent by calling 377-9174.