MINNEAPOLIS (WCCO) — The Minnesota State Capitol restoration makes the building look the way it did more than 100 years ago, when Minnesota’s best craftsmen built its most visible public landmark.

But this project is repairing a century of cover-ups and patch-overs. Project Manager Jason McMillan showed us skylights hidden for decades above false ceilings.

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“We knew they were there because you could see it from the roof,” McMillan said.

There are linoleum sheets over oak floors and ceramic tiles, and long forgotten “secret” doors, like one to the old Supreme Court balcony.

“A doorway there that just kind of led to nowhere,” McMillan said.

But most of the work here you will never see, even though it may be the most remarkable of all.

Workers are digging below the Capitol foundation to install modern air ducts, 48 inches in diameter, below the surface.

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And they’re hiding sophisticated heating, cooling, plumbing and power systems inside five-story vertical shafts behind the brick and granite walls.

“That goes all the way from the basement to the attic,” McMillan said.

Like most restoration projects, this one comes with unexpected challenges. Repair crews in the 1950s power-nailed copper to the outside Capitol dome. Instead of fixing the roof, they made it worse.

“Now, when we come to restore the roof, we take all that apart, we find that those pneumatic nailers actually destroyed the stone,” McMillan said.

It’s hard, dirty work. But these craftsmen say they feel a kinship to the ones who came before.

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The Capitol will have only limited public access during the 2015 legislative session — and it will shut down completely next summer until 2017, when it re-opens.