As the Smithsonian Institution in the U.S. celebrates its 175th birthday this year, the sprawling museum-and-zoo complex counts just over 155 million items in its 20 museums and off-site storage facilities. Across more than 11 million square feet of exhibition and storage space—most of it located in Washington, D.C., suburban Maryland, and New York City—those artefacts range from slingshots to space shuttles, ants to elephants.

It’s no surprise that only about one percent of the collection is on display at any given time. It's shared by many other grand institutions, such as London's Natural History Museum, which has some 80 million objects – of which only a 'tiny fraction' are ever displayed.

But that raises a question: What are we missing? As I discover when three Smithsonian museums kindly allow me to peek inside their back rooms, the answer is: lots of things that will absolutely blow your mind.

In the maze of storage rooms at the National Museum of American History, for instance, it seems every cabinet I look into holds an iconic cultural touchstone. Behind one door lies Ray Bolger's Scarecrow costume from The Wizard of Oz. A nearby drawer holds, side by side, Jerry Seinfeld’s puffy shirt and Mister Rogers’ red sweater. A small box contains the original stopwatch from the television news program 60 Minutes.

This treasure trove of popular American culture may not be exactly what James Smithson, a British scientist who never visited the United States, had in mind in 1829 when he left about $500,000 (£374,000) for the creation of “an establishment for the increase and diffusion of knowledge.” But there’s no denying that the institution that bears his name has come to be synonymous with boundless curiosity, relentless discovery—and, it appears, endless accumulation of stuff.

But don’t for one second confuse the Smithsonian with your grandmother’s attic. Meticulously organised and surprisingly selective, the museum’s archives are an essential resource in its mission to explore and preserve the natural and cultural wonders of America and the world.

When I ask Ryan Lintelman to open a tall, double-doored cabinet in the fifth-floor storage area at the American history museum, I don’t expect to find two of my oldest friends in there.

“Yes, that’s Mr. Moose and Bunny Rabbit,” says Lintelman, the museum’s curator of entertainment. For a moment, I’m speechless. In my childhood days, I spent virtually every morning with these two guys, Captain Kangaroo’s perpetual puppet foils. Bunny Rabbit still looks ready to steal a bunch of the Captain’s carrots any second now. But Mr. Moose, always the talkative type, has a thin white cloth tying his mouth shut.

“No, Mr. Moose doesn’t have a toothache,” says Hanna Bredenbeck Corp, collections manager for music, sports, and entertainment. “It’s just that without the tie his mouth hangs open, and that’s not good for him.”

I’m so enraptured by the flesh-and-blood—or fleece-and-cloth—presence of the pair that I almost don’t notice their companions on the cabinet shelf: the Muppets’ Swedish Chef, complete with apron and whiskers, and none other than Charlie McCarthy, the granddaddy of modern puppetry. In the late 1930s, this dapperly dressed hunk of wood was the single most popular performer on radio. He even won an Oscar.

Pulling open the top drawer of a case labeled “sports balls,” Lintelman points out a baseball signed by Babe Ruth. “A lot of these came from Ella Fitzgerald,” he says. “She was a big baseball fan.”

In another room we come upon an old library file cabinet. I assume it’s an obsolete office fixture, until I notice a drawer that reads: “Phyllis Diller.”

“Yeah, that’s Phyllis Diller’s joke catalog,” says Lintelman. “She was showing us things she wanted to give us at her home, and we were actually about to leave when we noticed this. She said, ‘Well, you wouldn’t want that, would you?’ Uh, yeah. Of course, we don’t take everything that people offer us. We’d never be able to store it all.”

I nod in agreement—and then I notice, in another glass cabinet, an exhaustive collection of lunch box thermos bottles: Dick Tracy. Kiss. Fireball XL-5. From this perspective, it’s hard to believe Smithsonian curators turn down anything. And yet they do. Every day generous offers of things like vintage yearbooks, classic toys, even back issues of National Geographic are turned away with polite letters of refusal.

Now we’re walking along rows of cabinets and boxes, opening anything that strikes our fancy. In a box labeled “Porgy and Bess,” there’s a full score signed by George Gershwin for 1930s dance band leader Milt Shaw. Through the window of a trophy case I spot a bronze fist—a cast of boxer Joe Louis’ hand. In a file cabinet of shallow drawers lie original animation cels of Mickey Mouse and Woody Woodpecker.

This could go on, literally, forever. But everyone has real work to do, so we reluctantly head for the door—where I spot, beneath Lance Armstrong’s bicycle, a crate labelled “Gremlin. Fragile.”

“Wait,” I say. “There’s a gremlin in there?”

“Yeah, I think so,” says Lintelman. He pries off the front and there, staring at us from behind a wooden brace supporting its head, is a perfectly preserved critter from Joe Dante’s 1990 horror comedy Gremlins 2: The New Batch. His eyes, peering over the brace, seem to implore us to set him free. But there’s a hand-printed warning: “Do not remove screws.”

I saw the movie. You don’t need to tell me twice.

“This is the world’s largest collection of sloth moths,” declares entomologist Alma Solis, holding a tray the size of a sock drawer. I’ve asked Solis to pull the drawer from storage unit U 29 in the Entomology Archive at the Smithsonian’s National Museum of Natural History. On this windowless level of the museum, rows of cream-coloured cabinets stretch in all directions toward distant vanishing points, like the world’s most expansive high school locker room.

Cabinet U 29 is marked “Pyralidae/Chrysauginae/Nearctic & Neotropical,” a category that includes teeny moths commonly known as sloth moths. Sloth moths, it turns out, are not so named because they’re slower than other moths. Nor do they have three toes. They do, however, have a symbiotic relationship with those slow-moving tropical treetop dwellers.

“Sloths spend most of their time in the trees, but they come down to defecate,” Solis explains. “Sloth moths lay their eggs in sloth faeces.”

The world’s largest collection of sloth moths numbers about 200, each one pinned neatly to a cushioned surface, each one assigned an individual number and a teeny tag that conveys a surprising amount of information regarding precisely where it was found, when it was found, and who found it.

That pattern of near-fanatical attention to order and detail is repeated drawer by drawer, cabinet by cabinet, row by row, as far as the eye can see—and beyond. With 35 million specimens, the Smithsonian’s entomology collection takes up much of the fourth, fifth, sixth, and seventh floors of the museum’s east wing.

“That’s about three football fields of storage space,” says Floyd Shockley, the entomology collections manager. “Plus, we have a comparable amount of space at our external support centres.”

That’s a lot of insects—beetles make up the largest group, if you’re keeping score—but they provide an essential database for tracking, for instance, changes in their habitat.

“We’re learning about how species distribution is changing with time,” Shockley says. “It can be caused not only by climatological changes, but also human-mediated habitat destruction and other things.”

Shockley gestures toward an array of insects he keeps framed near the elevator lobby. These terrifying-looking beetles, glamorous butterflies, and impossibly lanky walking sticks are “the things people expect to see here,” he says. “The cool, charismatic guys.”

But most of the diversity is in the small brown and black things, Shockley says. “I mean, take ants. If you were to add up all the ants on earth, they would outweigh all the vertebrates put together.” He seems thrilled at the prospect.

As we ride the elevator back down to the exhibition floors—home to crowd-pleasing dinosaurs and imposing whales—it occurs to me to ask Shockley if he ever feels bad swatting a fly.

“I try not to do it,” he shrugs. “But if there’s a spider in the house, and your wife wants it gone, well….”

While it’s true that highly qualified curators make tough choices regarding what items to display—based on the specific narratives they’re trying to relate in their exhibits—it doesn’t seem quite fair that so many treasures remain out of view. Why, you may wonder, can’t a museum make its off-floor acquisitions available to those of us who just want to wander around and see what’s there?

As it turns out, one Smithsonian facility—the American Art Museum, located a few blocks off the National Mall—has done just that. Tucked away at one end of the museum’s top floor is the Luce Center, an ornate, mid-19th-century gallery with two balcony levels overlooking a large, elongated space. Aligned on each of the upper levels stand rows of glassed-in storage shelves containing paintings, sculptures, carvings, pottery, folk art, and miniature patent models. It all resembles a well-organised and elaborate arts and crafts fair.

Here’s a case bristling with handmade walking sticks. Here are fever-dream “outsider art” paintings by Howard Finster, who believed his frenzied fantasies of apocalyptic destruction and soaring angels were directly inspired by God. (His “Vision of a Great Gulf on Planet Hell” is the stuff of nightmares.)

On a shelf of art deco sculpture models, I recognize a pair of burly men struggling to tame a team of rearing horses as the little brothers of “Man Controlling Trade,” two monumental statues located outside the Federal Trade Commission building, just a few blocks from here.

While there’s some rudimentary organisation—works gathered together by era, for instance—there’s almost no descriptive material. Most items are identified only by lengthy inventory numbers that visitors can research on the Smithsonian website.

Standing on the third level, I lean over the balcony railing and scan the numbered archive stacks on the opposite side. “Let’s see what’s down there at 12-B,” I say. “Oh, yay!” enthuses Eleanor Harvey, the museum’s senior curator. “That’s one of my favourites.”

Our footsteps echo as we descend the old metal staircase. I happen to have chosen an area largely dedicated to representations of Native Americans—almost exclusively by artists of European descent. In one cabinet hang portraits of Indigenous men and women, dignified and colourful in their traditional garb.

The painter is the famous travelling artist George Catlin. In five 1830s expeditions, probing as far west as Texas and North Dakota, Catlin created more than 600 images of Plains Indians. Nearly all of them now hang on this wall or are filed, unframed, in a wall-sized cabinet a few feet away. For researchers, they’re a treasure trove of otherwise lost detail.

Catlin traveled the U.S. and Europe displaying his portraits. Curator Harvey acknowledges these works reflect a larger issue facing the Smithsonian and other museums, which for too long rendered non-White people as little more than curiosities to be put on exhibit.

“There’s a lot of discussion right now about voyeurism and taking advantage of your subject,” Harvey says as we examine Joseph Henry Sharp’s 1906 painting, “The Voice of the Great Spirit,” depicting a Montana Crow chief’s platform burial. In the foreground stands a supposedly grieving widow—actually a woman named Julia Sun Goes Slow who posed reluctantly for the artist.

“I’m old enough to remember Natural History Museum dioramas that included Plains Indians and Inuit as if they were families of elk,” Harvey says. “There really was no distinction between the two. We’ve hired a Native American curator to help consult on how you tell the appropriate stories.”

Even with its open storage area, the American Art Museum still has about 70 percent of its collection hidden from view. And that’s okay with Harvey, whose philosophy echoes that of every other Smithsonian curator I’ve met.

“Some people say, ‘Well, you should just sell everything in the basement,’” she says. “But no, that undercuts everything that a museum is about. We’re here to tell the whole story of American art. And to do that we often can’t rely solely on the works on display.

“It may seem counterintuitive, but one of the things that makes a museum great is the stuff that doesn’t get shown.”