A locked door and myriad treasures beyond, in vast rooms deemed unusable at Boston Public Library

The resplendence of the Boston Public Library’s McKim Building unfolds with each step up the grand staircase.

Past the imposing stone lions, the golden-hued stairwell gives way to an airy gallery of murals, a millennium-spanning celebration of the muses. Nearby, a narrower stairway leads to the hushed third floor, where people wander the opulent gallery to untangle John Singer Sargent’s “Triumph of Religion,” a monumental cycle of murals the artist left unfinished despite nearly three decades of work.

Inevitably, some will try the leather-clad door at the end of the gallery, which seems to promise yet more wonders beyond.

But the door is locked.

It has been closed to the public for more than a decade, concealing a once-grand enfilade of vaulted ceilings, interior arcades, and elevate