In the cold waters of the North Sea, trawlers catch all manner of unintended things. Garbage, often. Porpoises, sometimes, for the shores of the Netherlands are teeming with hundreds of thousands of them.

But on May 30, several nautical miles off the Hook of Holland, a boat hauled up something perhaps never before seen by man. Two feet (61 centimetres) long and with two heads.

A photo of the thing began to circulate through the Netherlands - eventually to the inbox of Erwin Kompanje, the curator of mammals at the Natural History Museum in Rotterdam, who was astounded.

He recalled his first thought: "Where??"

Where was it, and how soon could he get it into a lab?

Even today, in an age of science, it might appear to many people to be a two-headed sea monster.

Kompanje knew better, though the truth was no less astounding. He was looking at photos of two conjoined harbor porpoises, newborns sharing a single body, a rarity among rarities in all the oceans in the world.

"There are only nine cases known until now" of conjoined twins among cetaceans, said Kompanje, who has studied dolphins, whales and porpoises for 20 years.

"Even normal twinning is rare; there's no room in the womb of the mother for harbouring more than one baby."

And the few specimens that had been found were usually undeveloped fetuses, like the 'double monster' - one head and two bodies - found in the womb of a dolphin caught by herders off the coast of Japan in 1970.

In 2001, a bottlenose dolphin calf with two beaks was caught in the Mediterranean Sea, but it was partially decomposed. And these are among the better-documented cases.

The photos Kompanje found himself staring at in shock showed a specimen that - while obviously dead - looked to be in perfect condition.

The brothers (for he could tell they were male) must have just been born, he reasoned, for the dorsal fin had not yet risen. And tiny hairs lost soon after birth could be seen on the two faces above the fin.

"I think it was born alive," Kompanje said.

The brothers couldn't have survived long. They would have either drowned as two brains told one body to swim different ways, or simply succumbed to their single heart's failure to pump enough blood.

But whatever happened in their short life, they had by pure luck found their way into a fishing net before nature destroyed their body's value to science.

Erwin Kompanje

For a researcher hungry for any morsel of information about man's aquatic mammalian cousins, it was a one-in-a-billion discovery.

So Kompanje got the boss of the fishing crew on the phone - providing the exact coordinates and details of the catch.

The possibilities expanded in the researcher's mind. Kompanje had been planning an MRI for the porpoises. A CT scan, 3-D imaging. And then, when the research was completed, the brothers might be preserved forever in a museum.

And then, on the phone with the trawler boss, he learned the true end of this strange tale.

"They thought it was illegal to collect it," Kompanje said.

"They made four photographs and threw it back into the sea. Back into oblivion."

In that expanse of deep water, there was no hope of finding it again.

Kompanje did what he could with what evidence was preserved. The fishermen's photos from the boat were good enough to co-author a paper with another marine researcher, which was published this month, and said what there was to say about the porpoises.

But Kompanje has no illusions. He does not expect to see anything like it again in his lifetime.

"For a cetologist," he said, "this a real horror."

2017 © The Washington Post

This article was originally published by The Washington Post.