Turtles migrate thousands of miles out in the open ocean, charting epic courses in search of food, mates, and nesting grounds. Exactly how they find where they’re going has long puzzled scientists who suspected magnetic fields were involved, but were unsure of the exact mechanism through which turtles were sensing it.

We’ve since learned that turtles appear to recognize magnetic signatures of locations, such as the beach on which they hatched where females will later return to lay their own eggs. We know the magnetosphere is in constant flux, and turtle nesting sites have been found to shift in tandem, so how is it that they’re able to make sense of this invisible force?

Some answers to this question were revealed in a study that looked at the way snapping turtles can tell north from south, in a phenomenon known as spontaneous magnetic alignment. It was once thought to be a rare trait in the animal kingdom, but as Professor John Phillips from the Department of Biological Sciences at Virginia Tech told IFLScience, this is no longer the case.

“Spontaneous magnetic alignment has been found in everything from cows to crayfish and all the animals in between. It’s not goal-directed, the way compass orientation is, or migratory orientation. It's basically that they’re lining themselves up to North and South.”

“It has to do – we think – with this really exotic sensory system, which is a quantum process that involves photoreceptors [whose] response to light depends on their alignment relative to the magnetic field.”

To explain the phenomenon, Phillips compares it to the weird visuals we experience after staring at a light source and moving our eyes away. That blur of complementary colors doesn’t exist in reality, but our fatigued photoreceptors perceive it anyway, giving us an image of something that isn’t physically there.

“Now, if that same kind of thing goes on with the magnetic field, then certain alignments in the magnetic field will [make photoreceptors] perceive light as being brighter or dimmer. And so, our guess is that they see this pattern superimposed on the world.”

It’s Phillips’ view that this supranormal visualization isn’t a goal-oriented map or a compass, but a sort of grid system that enables animals to organize spatial information. Rather than charting a course from point A to point B, the Earth’s magnetic fields helps them properly orient in a vaguely familiar environment.

The only problem is, we live in a world of increasingly prevalent magnetic fields, which can confuse matters.

“If you expose animals to very low-level radio frequency signals, it knocks out the magnetic compass,” Phillips continued. “And this happens in migratory birds. It happens in mice, in insects, it happens all over the place.”

This detrimental effect was evident in a 2015 study from Lukas Landler and colleagues (one of which was Phillips), that brought snapping turtles into the lab to explore how exposure to radio frequencies influences their capacity to orientate. It found that the turtles could align themselves towards magnetic north under experimental conditions, but could no longer do this after being exposed to low-level radio frequencies.

“What this suggests is that their perception, the pattern they see, depends on the electromagnetic environment, the radio frequencies that are present,” said Phillips. “So, radio frequencies can screw up the magnetic compass, because they simply don't recognize the new pattern.”

The Earth’s magnetic field is one of several cues that turtles rely on to make sense of the world. Celestial information helps hatchlings make it off the beach, but just like how increasing infrastructure is muddying magnetic inputs, human developments become a problem here too. Baby turtles are thought to head for the brightest horizon in search of the sea, but when natural light is outcompeted by the synthetic glow of a nightclub, they can go the wrong way.

The navigation skills of turtles evolved over hundreds of thousands of years in response to the natural world. Now that humans are changing the horizon, we may see migrating animals like sea turtles starting to lose their way.