337 endangered sei whales stranded themselves off the coast of southern Chile in one of the world’s biggest strandings ever recorded.

Sei, humpback and blue whales, which belong to the rorquals family, are the largest group of baleen whales, and “are not normally seen gathering in large groups,” which makes the stranding even more baffling to scientists.

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In this photo taken on April 21, 2015, and released on Tuesday, Dec. 1, 2015, by the Huinay Scientific Center, Sei whales lie dead at Caleta Buena, in the southern Aysen region of Chile. The coast of southern Chile has turned into a grave for 337 sei whales that were found beached in what scientists say is one of the biggest whale strandings ever recorded. (Vreni Haussermann/Huinay Scientific Center via AP)

“This is one of the largest strandings worldwide,” said Haussermann, the director of the Huinay Scientific Field Station, which focuses on marine research. She declined to disclose the conclusions, which will be published by a scientific journal later this year. h/t huffingtonpost

In this photo taken on April 21, 2015, and released on Tuesday, Dec. 1, 2015, by the Huinay Scientific Center, a dead Sei whale lies in Caleta Buena, in the southern Aysen region of Chile. The coast of southern Chile has turned into a grave for 337 sei whales that were found beached in what scientists say is one of the biggest whale strandings ever recorded. (Vreni Haussermann/Huinay Scientific Center via AP)

Based on their size and location, scientists believe they are all sei whales. The International Union for Conservation of Nature lists the sei as an endangered species.