A strange, undulating blob found in the waters of Ørstafjorden in Norway has turned out to be a rare sight: a giant mass of squid eggs.

The otherworldly ball, floating mysteriously in the darkness of the fjord, was discovered by captain Nils Baadnes and diver Ronald Raasch with the research vessel REV Ocean. It wasn't long before they learnt what it was.

"[It] is actually an eggmass of 10-armed #squid!" reads a tweet on the vessel's official Twitter account.

It's not known how squids produce these egg masses, but they are fascinating things: giant masses of mucus, sometimes metres across, inside which tens of thousands of eggs can be incubating. It's thought that the female lays a smaller mass that expands on contact with the water.

Different squids seem to produce different shapes of egg masses. For instance, the egg mass of the diamondback squid (Thysanoteuthis rhombus), found globally in tropical and subtropical waters, resembles a long tube of mucus with strings of eggs wrapped around it, like a slinky containing up to 43,800 eggs.

Humboldt squid (Dosidicus gigas) egg masses, on the other hand, are more like a transparent blob, as is that of Gould's squid (Nototodarus gouldi), the Japanese flying squid (Todarodes pacificus), and the neon flying squid (Ommastrephes bartramii), one of which measuring 4 metres (13 feet) across was filmed by National Geographic in 2015.

These masses are rarely seen by humans, because they tend to be laid relatively deep to start with. Then, they slowly sink lower and lower, to a depth of about 150 metres (500 feet), where the baby squids hatch, according to a National Geographic report.

The potential multi-purpose use of the mucus has not yet been fully explored (predator protection is a strong possibility), but evidence suggests that it serves as a protective barrier. As described in a 2012 paper, marine biologists attempting to raise squids in a laboratory using IVF found that the animals were prone to infection, and would die in a matter of hours.

It's unclear exactly which species of squid produced the Ørstafjorden mass. The REV Ocean Twitter account referenced the "10-armed squid", but since all squids have 10 tentacles, there is no squid with that common name. And it's often very hard to conclusively identify marine species just by the eggs, anyway.

Squids that live in the Norwegian Sea that the fjord connects to include the European flying squid (T. sagittatus) and the much smaller Boreoatlantic armhook squid (Gonatus fabricii). However, another Gonatus species, G. onyx, has been observed actually brooding its eggs, so it may be a less likely candidate.

Interestingly, G. onyx has also been observed pumping seawater into her egg mass to inflate it, so that may be another clue about how the masses get so big.

Overall, the finding - and our knowledge of squid eggs in general - is shrouded with plenty of unknowns. But whatever the species, the sight of the egg mass is certainly a magical one.