If you were living in the cold and rugged landscapes of medieval Scandinavia, exploring other places probably sounded like a good idea. Especially if it’s a place like the Azores — an archipelago of nine inviting islands in the middle Atlantic, some 1400 km (860 m) from Portugal’s coastline.

According to a new study, Viking explorers did just that: they reached the Azores centuries before Portuguese explorers. When the Portuguese came, they didn’t find any traces of the Vikings, but a new study detected “unambiguous” evidence that the Vikings were indeed the first on the islands.

In 1427, a Portuguese navigator set foot on an uninhabited, idyllic island. With its lush cliffs, beautiful beaches, and deep blue waters, the island must have been a sight to behold. We’re not really sure what the explorer’s name was — he is only known from a reference on a chart drawn by a Catalan cartographer in 1439, and the map was marred by an inkwell accident in 1869 that partially smudged the explorer’s name.

Historians suspect that his name was either Diogo de Sunis or Diogo de Silves — but whatever it was, he must have thought he discovered a new part of a previously unknown land. Later Portuguese explorers confirmed his finding and revealed it to be a part of the Azores archipelago, nine volcanic islands.

Nowadays, the Azores islands are as dramatic as ever, but they’ve been settled by the Portuguese for centuries. However, some researchers suspected that this may not be the whole story. Particularly, that someone (Vikings) was on the island before the 15th century.

But since there was no archaeological evidence (that was found yet, at least) to support that idea, they had to look for evidence in other places.

In 2015, a study found some evidence in the unlikeliest of places: mice. The study noted intriguing genetic similarities between mice in the Azores and in Northern Europe. But as tantalizing as this evidence was, it was insufficient to draw any clear conclusions. Now, the needed evidence may have finally been discovered.

Around a decade ago, Pedro Raposeiro, an ecologist at the University of the Azores, Ponta Delgada wanted to explore the Azores’ climate by analyzing sediment cores from lakebeds across the archipelago. This is a common approach used by many climate scientists, and the layers of sediment from the cores can be dated pretty accurately.

But in addition to climate information, researchers also found signs of human disturbance: pollen from non-native crops that explorers would have brought along and spores from fungi that grow on livestock dung. This was not surprising — but what was surprising was that these traces extended all the way back to 700 years before the Portuguese settled on the Azores.

In one particular layer, that was dated to AD700-850, researchers found clear signs of human activity: an increase in charcoal particles, a dip in the pollen of native trees, and a compound (5-beta-stigmastanol) that is found in the feces of animals such as cows and sheep. This suggests that some of the island’s trees were being cut down and burned, presumably to make way for pastures.

Similar signs were found in a layer dated to 100 years later, as well as in layers dated to 1150 and 1300 respectively.

“The occupation of these islands began between 700 and 850 CE, 700 years earlier than suggested by documentary sources. These early occupations caused widespread ecological and landscape disturbance and raise doubts about the islands’ presumed pristine nature during Portuguese arrival,” the researchers write in the study.

The team adds that the Norse were likely the ones who first set foot on the Azores. They were among the few who had the technology to reach the islands (or perhaps the only ones), and by the 8th century, various areas in Europe noted that they were reached (and attacked) by Norse seafarers. While there’s no smoking gun pointing to the Vikings, they are the most likely culprits.

“These results are consistent with recent archaeological and genetic data suggesting that the Norse were most likely the earliest settlers on the islands.”

So what happened to them? When the Portuguese arrived in the Azores, they described the islands as “pristine” and said they found no trace of anyone, so the previous settlers had already left for some time, for reasons that are not entirely clear. That’s a story for another time.

The study was published in PNAS.