By day, Lillian Egner works as a program manager for the Soap Factory, an experimental art space in downtown Minneapolis. A typical week for Mike McGinley, who works at St. Croix Sensory, his family’s business, involves odor-testing kitty litter and training environmental-protection officers in the correct use of the Nasal Ranger, a nose-mounted, megaphone-shaped “field olfactometer” invented by his father to measure smell. But, every October for the past eight years, Egner, McGinley, and a crew of artists and volunteers have come together to create a custom smellscape for the Haunted Basement.

Housed directly beneath the Soap Factory galleries, in the building’s grimy, raw underground space, the Haunted Basement consists of a series of rooms, or scenes, each created by an emerging artist. Despite (or perhaps because of) its highbrow origins, it’s generally agreed to be the freakiest haunted house in town—only adults are allowed in, dressed in closed-toe shoes and a protective face mask, and armed with a safe word (“uncle”), just in case. Visitors enter in groups but often get separated as they move through the twelve-thousand-square-foot space; they can expect to crawl, climb, and run, to get covered in gore, and, in 2013, to be stuffed into a coffin by a toothless man in an orange jumpsuit. The entire experience lasts a brief but intense twenty minutes, though nearly a hundred people bailed out early this year by crying uncle.

Over the years, haunted-house designers (hauntrepreneurs, as they sometimes call themselves) have developed a finely tuned vocabulary of architectural, lighting, audio, and animatronic techniques to provoke fear. A twisting, maze-like floor plan, for example, induces disorientation and cuts down sight lines, increasing the opportunities to deliver a “startle” scare—that pants-wetting moment when a zombie pops out of nowhere. Careful lighting creates pockets of shadow in which actors can hide; a soundtrack can shock, distract, and contribute to over-all discomfort; and fog machines generate a reliably creepy atmosphere. Rubber limbs, fake blood, and animatronic ghouls add the appropriate touches of visceral horror. But smell, despite its ability to affect mood, enrich narrative, and add authenticity, is not usually part of the package.

That’s partly because odor is extremely hard to deliver effectively. The physics governing sound and light is much better understood and easier to manipulate than the physics governing volatile aromatic compounds, which disperse and persist in ways that are difficult to predict. Early experiments with theatrical scenting, such as Hollywood’s Smell-O-Vision and AromaRama systems, were critical flops, with audiences complaining that smells took too long to reach them and were either undetectable or too strong—or, worse, failed to disperse on time, accumulating into muddy, headache-inducing clouds.

Today, fifty-four years after Smell-O-Vision’s underwhelming début, episodic smell delivery is still an unsolved problem. Mike McGinley, a chemical engineer by training, was drawn to it. In 2007, he read an interview with the Soap Factory team in a local paper, midway through their first Haunted Basement production. “It’s a different type of haunted house being put up by artists to raise money for an art gallery,” McGinley said. “I read that and I thought, Boy, that sounds really cool. Then I got to this line about how they were striving for a multisensory experience. And I sat there going, I think I need to make a phone call.”

One cold call later, McGinley and the St. Croix Sensory squad (known affectionately by Egner as the Nasal Rangers) became a permanent part of the Soap Factory’s haunting team. “I absolutely couldn’t imagine it without them,” Egner, who has been with the Soap Factory since the very first (odorless) Haunted Basement, told me. “It makes it so much easier to manipulate visitors, because you can’t guard against your perception of smells. It’s very powerful.” Meanwhile, for McGinley, the collaboration has offered a fun excuse to conduct research on odor dispersal, as well as the chance to tackle some of the challenges that have stopped smell from being an easily deliverable media format.

The first year, McGinley said, “we started simple. I believe it was five smells we utilized, in maybe seven different areas of their basement. You know, fairly basic smells—death and body odor and things like that.” It turns out that death, in odor form, is indeed straightforward: a couple of relatively coöperative naturally occurring chemicals, putrescine and cadaverine, are responsible for the characteristic smell of a decaying corpse. McGinley simply orders a small quantity of each from a chemical-supply store every September, and they show up in the mail.

The cadaverine has since become a trademark—both artists and audiences apparently complain if the Nasal Rangers try to leave it out. But, from that base, the Haunted Basement odorscape has greatly increased in complexity and nuance. “This year, we developed twenty-four different smells for the whole space,” McGinley said. A collaboration that used to start in late summer now continues year round, as McGinley builds a mental catalogue of particularly disgusting smells encountered during the course of his regular odor-analysis work. One commercial project, which involved testing a new air freshener for its effectiveness against various household smells, resulted in a “garbage recipe” that McGinley used in a Unabomber-style trailer scene in the following year’s Haunted Basement. “Some things we were learning through that process helped us understand how smells of garbage might work real well for certain situations in the basement,” he said, with the tone of a man who has smelled far too much garbage.

The artists, meanwhile, develop a new layout and story line each year, which often leads to unexpected challenges. Once, McGinley said, there was a scene that called for a persistent, low-level gasoline smell. “Obviously, we couldn’t just aerosolize gasoline.” Even after he substituted a motor-oil smell formulated from slightly less explosive chemicals, it proved very difficult to get the odor to last long enough at a low enough level. “It was one of those smells that very quickly became overpowering, but it also wasn’t persistent,” McGinley said. For that particular scent scene, he ended up having to design and build a custom “aeration bubbler,” which pushed a steady stream of air through a scented liquid and then diffused it, laden with aromatic compounds, into the room.

Alongside smell-delivery systems, the team has developed an ever more refined sense of the narrative role that smell can play in a haunted house. “Initially, it was just bad smells,” McGinley said. “By the second or third year, we began to wonder, O.K., what else can you do with smell that is disorientating or unexpected?’” That year, there was a scene set in a nursery—“let’s just say that unsettling things were going on,” McGinley said, when I asked for more detail—and the team added the smell of baby powder at the entrance, hoping to trigger the warm and fuzzy associations that visitors might have with that scent, to lull them into a false sense of security.

Egner’s personal favorite haunted scent experience was another bait-and-switch scene: as visitors approached a dimly lit table, they could smell roast turkey, mashed potatoes, and stuffing, and see dishes piled high with food—which turned out, on closer inspection, to be cured hog guts and other viscera. “I’m sure that we ruined somebody’s Thanksgiving that year,” Egner told me gleefully.

The Soap Factory does make visitors sign a waiver before they enter the Haunted Basement. “It’s intense,” Egner said, adding that, most years, one or two patrons vomit during their visit (requiring a sanitary cleanup and adding an extra layer to the smellscape). McGinley admitted that he himself hasn’t always seen the basement in action. “It’s not the smell—it’s the other stuff that freaks me out,” he said. But the only time that a smell has been toned down midseason was this year—and it was for the benefit of the staff rather than the visitors. “There’s a really strong cat-urine smell this year, and it is really sticking with us,” Egner said. “Our clothes, our hair … I have a dog, and he is very confused when I get home.”