In my imagination an ancient Sumerian farmer is getting tipsy. There he stands, tripping over his sandal-clad feet, smiling and hiccupping as he waves to bewildered neighbors. He’s been drinking from a pot of sour-smelling water, inside of which some misplaced bread slowly disintegrated over the course of a season. It’s fun to think about the origins of beer. We’ll never know what really happened, but for thousands of years beer lovers believed the fermentation process was some alchemic miracle (or, as Benjamin Franklin put it, “proof that God loves us…”).

That all changed when Louis Pasteur discovered the existence of yeast, allowing brewers to isolate the particular yeast strain he or she wanted to use, ensuring a consistent brew as long as they kept out the wild yeast floating in the air. As a result, the beer industry now uses lab-reared cultures and a sanitized workspace fit for a scientist.

This drive for sterile conditions and consistency is now, like an elastic, snapping back the other direction. The rise of craft beer in the United States has awoken a search for authenticity that is leading people like Rob Tod, owner of Allagash Brewing Company in Portland, ME, back to square one. Tod is among a cadre of brewers throwing sanitation aside to experiment with that alchemic miracle again. In their quest, Tod and others are re-examining the Belgian lambics, a style of beer made by exposing the hot wort to the open air – a sin in a modern brewery. Lambics are made exclusively by a small group of Belgian brewers who make them the same way their forbearers did for hundreds of years.

I recently visited Tod at his brewery to sample his first batch of experimental beers made in the lambic style. The brewery is expanding to meet demand for its flagship brew, Allagash White, but inside a small building attached to the brewery is a large, shallow, metal tub. This open-air container—known as a koelschip in Belgium and a coolship at Allagash—was the original heat exchanger. It cools the hot wort, but also allows wild yeast and bacteria to inoculate the mixture before it is added to the barrels and the fermentation process begins. The process, called spontaneous fermentation, produces a sour beer, which some say is an acquired taste. “I always tell people: ‘when you taste this beer, you’re tasting 5,000 years of history,’” says Dan Shelton, president of Shelton Brothers, one of the major U.S. importers of Belgian lambics.

The lambic style fell out of favor in the 20th Century. “The style just faded away,” Tod says. “That’s how beer tasted for thousands of years, and now all of a sudden when sterile heat exchanges came along… people’s preferences changed.”

Tod doesn’t call his spontaneous-fermented beers lambics out of respect for his Belgian contemporaries. Instead, he calls them “spontaneous beers” or “coolship beers” and says they are “an American interpretation of the style.”

As far as Tod knows, Allagash Brewing was the first modern brewery in the United States—if not outside Belgium—to build a traditional coolship. In 2009, not long after it was completed, The Atlantic called Allagash’s coolship “the future of American craft beer.”

After visiting the coolship, Tod leads me into the brewery’s inner sanctum. The room is refrigerated, French and American oak barrels are stacked to the ceiling. Among the towers of barrels, one upturned barrel serves as a table and carries three unlabeled bottles, each of which contains a beer modeled on a different traditional lambic style—gueze, kriek and framboise.

Tod cracks open the gueze first. Gueze is a mixture of three-year-old lambic, one-and-a-half-year-old lambic and young lambic, Tod explains. (Blending, he says, “is a whole art in itself.”) The beer was crisp and sour, not unlike a traditional gueze. This one will be sold as “Resurgam,” which is Portland’s motto and means “I shall rise again.”

“This is how they made beer for thousands of years,” Tod says. “They’re tart, they’re different.” But after the style fell out of favor, people weren’t exposed to it. “And since they’ve never traditionally been brewed here, the common thought was you could only make them in the Senne valley area in Belgium.”

Yet there is no special yeast in the Senne valley specifically suitable to make spontaneously fermented beer. Allagash has shown that spontaneous fermentation can produce tasty beer anywhere. But like Tod tells me, they didn’t set out to disprove any myth or tear down the Belgian brewers. They were just curious to see if it would work. And it did.

“I have personally always believed that spontaneous fermentation can and will happen all over the world in all different forms,” Jason Perkins, Allagash’s brewmaster, told me. “The key with what we did is that it turned out well – the beers are not only drinkable, they are delicious!”

The kriek is next. To make kriek, brewers add cherries to the lambic after it has fermented in barrels for a year and a half. The lambic is left to age with the cherries for a few months, then bottled. Allagash’s kriek-inspired brew is a lighter red than traditional krieks. This one will be sold as “Cerise.” “We’re not trying to brew beers exactly like their beers,” he says. “We’re doing our own thing, but this does not taste unlike the traditional Belgians.”

The framboise, lambic aged with raspberries, is next. It’s a deep red, fruity with a delicious tartness. This one the brewery named “Red.” So far, this is the only coolship beer Allagash has released for general consumption. Allagash on Aug. 31 quietly placed 400 bottles in its brewery store, along with specially designed glasses. They sold out in less than 24 hours.

Allagash’s recent experiment was a “huge expense,” Tod says. In fact, the entire room we’re standing in is full of experiments – aside from about 10 barrels of Interlude. “We basically don’t sell anything in this room and never really have for three years,” he says. “If an investment bank bought us, the first thing they’d do is come in here and be like ‘so you’re telling me you don’t sell any of this stuff and you’re just experimenting and learning?’ They’d be like: ‘This stuff goes. You’re putting fermenters in here.'”

But despite the cost, Tod says it’s a good investment. “This room is less than half a percent of our volume, but 50% of our culture. This is a very important room here. Experimentation, innovation… that’s what these beers were all about.”

Tod takes another sip of his raspberry-infused Red. “I know I’ll be taking a lot of this beer home to drink,” he says. “I just love this beer.”