Thick, full of protein, and allegedly very good for you, skyr is a relative newcomer to the shelves of American grocery stores. It’s been made in Iceland for more than a thousand years, so we’re a little late to the game.

In America, skyr is found in the yogurt aisle and is labeled as yogurt. However, if you are lucky enough to visit Iceland, people there will tell you that skyr is not a yogurt at all, but rather a cheese.

So which is it? Is skyr a cheese or just another strained yogurt presented in a sleek, Nordic package? The answers to these questions reveal there’s much more to skyr than meets the eye.

To learn about skyr from someone who knows their skyr, I called Gunnar Karl Gíslason, acclaimed chef at New York City’s Agern restaurant and author of North: The New Nordic Cuisine of Iceland. His earliest memories of skyr are with his grandfather, who ate very sour “old” skyr for lunch every day. To Gíslason, the best way to eat skyr is simple: cold with cream and fresh fruit.

Skyr is a central part of the Icelandic diet and is intimately tied to Icelandic culture. It was likely brought to Iceland by the Scandinavian Norsemen who settled the land in the late ninth century. Like the beautiful Icelandic horse, skyr disappeared from the rest of the Nordic region, but in Iceland, it continued to evolve into the unique product it is today. Traditional skyr is not a static product. It changes in flavor over time, once it is made and packaged for consumption. This is because the microbes used to ferment the skyr are still active, converting lactose (the sugar found in milk) into lactic acid (the molecule that lends yogurt and cheese their acidity) and other flavorful products. “Back in the day,” Gíslason said, “skyr didn’t come in fancy buckets like it comes now.” Rather, slabs of skyr were wrapped in plastic and piled into trays as the whey continued to drain out of it. At the front of the stack they’d have the new skyr—“the baby skyr.” The old stuff would be in the back. “That was for the real people,” he said. “It was much more sour.”

To help preserve the history of skyr, a Slow Food Foundation for Biodiversity presidium, or small research group, formed in 2015 to study the processes and ingredients of traditionally produced skyr, as well as to promote interest in Iceland. I asked the coordinator of the presidium, Dominique Plédel Jónsson, about the traditional skyr-making process.

The traditional recipe for skyr involves taking milk (skim or low-fat) and heating it with a bit of old skyr from a previous batch, which is added as a starter, Jónsson explained. Rennet may also be added, and after curds form, the whey is drained slowly for many hours to create a thick, sour product.

Rennet? That sounds a lot like cheese.

But what about the skyr sold in the United States? Are these products the same as the traditional Icelandic version? Are they pale imitations or modern takes on an ancient food?

I spoke with Smári Ásmundsson, CEO and founder of Smári Organics, an American skyr company distributed nationally. He said their recipe was developed from the knowledge accrued from a decade of home skyr making and many long conversations with an expert Icelandic skyr maker (and family friend). The Smári Organics recipe follows the same basic outline as the traditional recipe, with one important difference: Half of their products are made from whole milk (rather than skim or lowfat). Ásmundsson likes to call his product Icelandic yogurt, instead of skyr, for this reason. “After a thousand years of making skyr,” he said, “we revolutionized skyr making by making it with whole milk.” In addition to their whole-milk products, Smári Organics also sells more traditional skyrs made with skim milk.

It certainly makes sense to market skyr as a yogurt in the United States. “Yogurt’s doing very well,” said Jim Carper, editor in chief of the industry publication Dairy Foods. In recent years, the yogurt industry has seen significant growth: The U.S. Department of Agriculture’s National Agricultural Statistics Service reported an increase in yogurt production from 4.1 billion pounds in 2010 to 4.7 billion pounds in 2014.

According to Carper, the arrival of Greek yogurt on the scene helped to drive that growth. “When Greek yogurt took off in popularity, that really, really raised the profile of cultured dairy, yogurt-type products.” In 2007, Greek yogurt had a measly 1 percent share of the yogurt market; today it commands more than 50 percent. Consumers are drawn to the protein-rich items in the dairy aisle, not only because they taste good, but also because experts stress their nutritional value.

Most skyrs contain more than 20 grams of protein per cup, with little to no fat or added sugars. And to pioneering skyr makers in America, health concerns are both motivators as well as useful marketing tools.