Marine biologists in Brazil were shocked to find a dead humpback whale in a mangrove on Marajo Island, in the Amazon River, last Friday.
Government officials told news outlets that the 8m-long whale had been dead for several days.
While tens of thousands of humpback whales spend part of the year feeding off the coast of Brazil, they migrate south during this time of the year to feed near Antarctica.
Members of the conservation group Bicho D'Agua found the whale after following vultures that were circling overhead.
"We imagine it was floating and the tide took it into the mangrove," Dr Renata Emin, president of Bicho D'Agua, told Brazilian news site G1. "The question is: What was a humpback whale doing in the month of February on the northern coast of Brazil? It is unusual."
Scientists examined the carcass at the weekend to figure out how the whale died and took samples for a necropsy, the group said.