A pod of killer whales has sunk another yacht off northwestern Spain, reinforcing a bizarre pattern of vessel encounters that has stretched on for more than six years along the Iberian coast.

In the incident, reported Friday (July 24), the whales destroyed the rudder of a 46-foot-long (14 meters) Swedish-flagged yacht off Galicia in northwestern Spain, forcing the two people aboard to send a mayday call. A Spanish coastguard vessel towed them to safety, Spain-based news outlet Sur in English reported.

The incident came after several other encounters in the same waters earlier that week, including one in which a pleasure craft called the Idem sank near Estaca de Bares, the northernmost point of the Iberian Peninsula. The ship sank after the killer whales, also known as orcas (Orcinus orca), destroyed its rudder. Both occupants were rescued.

That makes the Idem at least the eighth boat lost to these encounters since they began, according to groups that track the incidents. More than 700 interactions between orcas and vessels have been logged since 2020, ranging from a brief nudge to a multihour rudder-ramming session, according to the Atlantic Orca Working Group, the Spanish-Portuguese research collective that has coordinated data collection on the phenomenon from the start.

Researchers have identified roughly 15 individual killer whales involved in these incidents, out of an Iberian subpopulation of fewer than 50. The group is so small that the International Union for Conservation of Nature lists it as critically endangered, as it is dependent mainly on Atlantic bluefin tuna for its food.

The family-like cluster is thought to be led by a female known as White Gladis. That name comes from an early theory researchers themselves once floated: that White Gladis suffered some kind of traumatic run-in with a boat and that the encounter flipped a behavioral switch, with her being the first in the group to start ramming rudders.

Alfredo López Fernandez, a biologist with the Atlantic Orca Working Group, told Live Science in 2023 that this kind of defensive, trauma-driven origin was gaining traction among researchers at the time. But "defensive" isn't "vengeful" — and the idea has since curdled online into a punchier revenge story most scientists studying the population don't endorse.

So why do orcas go after the boats? Nobody knows for certain, but the leading explanation is simple: Orcas are some of the most social animals on the planet, and social animals pick up habits from one another the way trends spread through a friend group. Researchers describe the behavior as socially learned — young orcas appear to watch adults ram a rudder and then try it themselves, and the habit has spread through the group over time.

A 2022 study in the journal Marine Mammal Science found that the killer whales usually targeted sailing boats and often lost interest after striking the rudder and causing the boat to stop.

None of that makes the encounters any less disruptive for the people living through them. The standard guidance from authorities like the Maritime Safety and Rescue Society and the Atlantic Orca Working Group, refined over repeated encounters, is to cut the engine, drop the sails if possible, disengage the autopilot, and let go of the wheel entirely, giving the orcas nothing to push against. Boats that follow this protocol tend to sustain less damage than those that don’t.

Sailors are asked to report every encounter. Each one adds to the data scientists are using to try to solve a years-long mystery.