Late in May 2017, Dutch fishermen caught two-feet long and a two-headed harbor porpoise in a beamtrawl net while fishing in the North Sea.

Afraid that keeping the deceased anomalous creature might be illegal, the fishermen threw it back in the ocean. However, they made sure they took pictures of the marine mammal for researchers to work on it. And in that vast sea, there is no hope of finding the creature again.

Such cases of two-headed creatures are seldom seen in wild mammals. History confirms that only nine other cases of two-headed cetaceans have been documented before this discovery.

The study recently published in Journal of the Natural History Museum Rotterdam scrutinized the uncommonness of the newly discovered creature.

Erwin Kompanje, the author of the study stated, “Descriptions of conjoined twins in whales and dolphins are extremely rare. We were aware of only nine [other] published cases.”

“They thought it was illegal to collect it. They made four photographs and threw it back into the sea. Back into oblivion,” said Kompanje, per National Geographic.

Though being unable to physically inspect the porpoise, the unhappy researchers believe that they could possibly garner facts from the fishermen’s pictures of the bizarre creature. However, one thing the scientists are sure of is that the porpoise was a male and a newborn, reported The Washington Post.

Through the picture, the researchers observed that the dorsal fins were not yet erect and the rostrums (the top of its heads) still had hair on them. Also, the creature still contained an umbilical opening and the tail had not yet stiffened, which is important for porpoises to swim – these factors signified that the porpoise passed away shortly after its birth.

Per studies, porpoises can produce only one offspring every one or two years and even then the cases of twins are very rare. The International Union for the Conservation of Nature pointed out that the greatest threat porpoises face is their incidental catch. Thus, the European Union has taken measures for monitoring the effect that nets trawls have on populations.

Usually symmetrical conjoined twins like this porpoise are believed to be as a result of two separate embryos fusing together or a zygote only partially splits. However, Kompanje noted that what caused the conjoined twins still remains a mystery.