John Mertz, of Wanaque, N.J., looks at a display at Gettysburg National Military Park Museum and Visitor Center on Monday in Gettysburg, Pa. (Matt McClain/The Washington Post)

Behind the metal door of Room 133, where a sign says, “Enter, Alarm Is Off,” lies a trove of objects that the hallowed ground here has given up over the past 150 years.

Hundreds of old muskets, bullets, swords, saddles, bullet-pocked furniture, ceiling gasoliers, objects in boxes and drawers, objects covered now in ghostly white muslin, have been collected, after being dug up or found hereabouts since the shooting stopped.

And more than a million of those items reside behind the door to Room 133, in the lower level of the museum at the Gettysburg National Military Park’s Visitor Center.

On Monday — the 150th anniversary of the first day of the historic three-day Civil War battle — Greg Goodell, museum services supervisor, opened its inner sanctum to show off some of the rarely seen treasures.

The park’s new state-of-the-art museum upstairs displays only about 1,300 of the best artifacts in the collection, Goodell said. It was thronged with visitors Monday for the battle’s sesquicentennial.

The rest of the items are kept in cool, dehumidified storage on shelves and in filing cabinets below.

The large room is a spare, sterile, unadorned environment of shiny metal that contrasts with the browns and grays of the artifacts.

“Every piece back there has some sort of story,” Goodell said.

The Park Service is not generally known for having important historical repositories. But “we have probably the most significant, most comprehensive Civil War collection in the public domain,” Goodell said.

“This collection started to be gathered as soon as the guns fell silent, so it’s been building for 150 years,” he said. “It started out with a number of private collectors in the area actually going out on the battlefield to . . . gather pieces after the . . . battle finished.”

Indeed, resident Liberty Hollinger wrote in a memoir of residents and visitors poring over the battlefield “to pick up anything of value. . . . Blankets, sabres, and guns, and many other articles were thus obtained.”

“The largest collection that came into the National Park’s possession is a collection called the Rosensteel collection,” Goodell said.

John H. Rosensteel was a 16-year-old Gettysburg native who, while assisting burial details two days after the battle, saved a dead Confederate’s musket, according to a history of the battle’s aftermath by Gregory A. Coco.

Rosensteel and his relatives later established here one of the best private Civil War museums, with about 30,000 artifacts. The Park Service acquired them in 1974.

Much of the older material still has old-fashioned museum display designs and markings.

Dozens of bullets are arranged on display boards in fancy circular or diamond patterns. An old corner cupboard is marked with an arrow drawn on a piece of paper pointing to a bullet hole, so the viewer couldn’t miss it.

Another antique label states in handwritten black ink, “Acorns from the ‘Clump of Trees’ at ‘High Water Mark’ Gettysburg 1899.”

It refers to the grove of trees that was the aiming point of the doomed Pickett’s Charge, the Southern attack on the last day of the battle that is considered the “high water mark” of the Confederacy.

It’s not clear if the acorns survive behind the label, but someone considered them historic enough to save, even three decades after the battle.

The park’s collection also includes a mint-condition 1859 McClellan cavalry saddle, with its wooden “tree,” or main structure, covered in sheepskin. Its leather saddlebags, stirrups and “boot,” which held the tip of the rider’s carbine, are intact.

It also has a large piece of the six-foot-long silk swallow-tail guidon that probably marked the headquarters of the Union Army’s Second Corps, commanded by Maj. Gen. Winfield Scott Hancock, during the battle.

Gold and silver gasoliers — decorative gaslight fixtures — that came from a house in Gettysburg during the battle are carefully stabilized with white ribbons. Bed boards from Gettysburg houses are stacked atop filing cabinets.

“There’s a couple bullet marks in some of them,” Goodell said. “They would have been damaged during the battle.”