When an old building gets rehabbed, old secrets see the light of day again. A faded sign, a stairwell walled off for decades, elegant carvings long covered by a suspended ceiling.

If you passed the ordinary office tower at 5th Street and Marquette Avenue S. in downtown Minneapolis recently, you may have glimpsed some ancient steel beams anchored by enormous rivets. It’s not unusual — now and then a building gets stripped and its skeleton shows. They slap on a new skin and it’s good for another 50 years.

But those beams were the bones of a building Minneapolis fought hard to get: the Federal Reserve. St. Paul wanted it, but when area bankers studied each town’s proposal, Minneapolis won 365 votes to 93, and the Ninth District Fed was incorporated in 1914.

At first it was housed in offices in different buildings, including the New York Life Insurance building and the Lumber Exchange. By the start of the 1920s, a consolidated location was proposed, and who better to design the prize than Cass Gilbert? He was the great Minnesota architect who bestowed the Twin Cities with lasting gifts. The State Capitol. The great broad mall of the University of Minnesota.

You could say that each is a theatrical set for the dramas people play on their stages, and that they ennoble the participants with their historical vocabulary. You may be arguing about a petty piece of legislation or walking to class with a hangover, but the monumental style should take the slouch out of your posture, remind you that history didn’t start last week. Gilbert was our great classical architect, and he left us public spaces that ennobled the city.

Except for the Federal Reserve.

Opened in 1925, it was the meanest, haughtiest, most forbidding thing Gilbert ever did, and that was probably on purpose. When you look at old photos of its blank stone walls, you wonder if it was designed to make stupid criminals think twice about busting in with guns barking. It was impregnable. It was Gibraltar. It was the pride of Minneapolis, and it was a stone sponge that soaked up the energy of the street and glowered out a warning: Go away.

It had no windows. If you managed to batter down the front door and make it to the safe, well, good luck. The massive door to the money room extended below the floor through an ingenious system of pivoting plates and tumblers, so you couldn’t roll a cylinder of gas under the gap.

The door is still there. The mechanism for locking the huge vault is still in place, with a plate that bears the name of Frederick S. Holmes, its designer. In a final touch that almost seems like an ingenious act of spite, there’s a door behind the vault door with a combination dial, like a gym locker. Even if you got through a yard of thick nickel, you still had another door to get past.

The workers reconfiguring the space for its 21st-century role have to work around the door. Everything else connected to the structure’s purpose may have moved away, but the door’s not going anywhere.

Eight stories added

By the 1950s the Fed needed more room, and decided to pile eight stories on top of the old building. It was classical — a churlish example of the style, sure, but it was columns and pediments and scrollwork, and no one did that anymore. Solution: Stick a square metal tower on top of it and call it a day.

Hence: the most confused building in downtown Minneapolis. Imagine the IDS rising out of the Basilica, and you get the idea. The renovation was finished in 1955, and the building lived out this fish-and-fowl existence until the Fed decamped for its new home in 1972. A few years later the classical facade was pried off and replaced with a stone grid that was as boring as the office tower, but in a different boring style.

At least that’s what it looked like outside. On the second floor was one of downtown’s most intimate and civilized hidden spaces. The skyway level had an atrium with a stone maze in the center, a warren of benches and plants with a stream that followed in square channels. Eventually the plumbing faltered and the stream was filled with rocks, but even then it was a quiet place, a tidy oasis where once had been marble, money and guards with guns.

A beautiful home

The Federal Reserve has relocated twice: first to Gunnar Birkerts’ ingenious building that suspended the floors from two towers like the deck of a bridge. It’s a beautiful building, but it was later cursed with an addition that feels like a backpack full of bricks.

The third Federal Reserve is generic, and while it could rest in any asphalt sea in a suburban development, its curving facade is a nod to the river, and its clock tower a nice piece of civic charity.

You might think the second Federal Reserve was a government building, because it doesn’t seem like anything anyone would design if rental income was the objective. But the first one? Federal Reserve, no doubt. It said BANK, but it also said BUZZ OFF.

No more. The renovation underway, shaped by local architect David Kelly, eliminates the dated skyway atrium and adds new office space. The first floor is waiting for a restaurant, or retail. The revision of the lower floors opens up the space and lets you look inside. It’s the opposite of what Cass Gilbert intended, but you suspect he’d approve. If it’s not going to be the Federal Reserve, then let it shine.

The vault, as noted, is still there — and the door is open, in case they want to store anything again. Come home. All is forgiven.