Several years ago, when trying to come up with ways to entertain small children on a chilly winter day, I decided to purchase a heavy-duty magnet. A friend of mine had given me the idea to attach it to a line and try dropping it through cracks in the ice along the shore to see what we might pull up.

We trekked down to the banks of the Androscoggin one evening around sunset to see if we’d have any luck. This led to a discussion of what kinds of things might stick to the magnet and what wouldn’t. But, what I hadn’t thought of until recently was whether there was magnetism in the water itself.

It turns out that freshwater is actually essentially “anti-magnetic.” The proper term that describes water’s counter-force is “diamagnetic.” This is not the case for saltwater, which is a conductor of electricity, and thereby generates its own magnetic field in the presence of a magnet. Because Earth has its own magnetic field, when sea water moves in a significant way, the result is slightly “charged” water. It isn’t as if your hair would stand on end if you went swimming, but this weak charge is detectable enough that oceanographers can measure it using satellites. This lets them create images of major water movements like currents and tides. It’s neat to think that the gravitational pull between the Earth and the moon, which generates the tides, generates a small but measurable force in the water itself.

Apparently, these magnetic measurements can also indicate the ocean’s temperature. That is because higher temperatures result in a weaker magnetic field. By tracking differences in the magnetic signal, scientists can track changes in temperature. They can do this far into its depths of the ocean — and even below it since the seafloor is closer to the Earth’s magnetic core than its surface.

One of the amazing things that Earth’s magnetism allows us to do is to navigate. A compass uses this magnetism to help determine the basic points of direction. Marine life use these same orientation aids underwater. They have specialized cells that allow them to sense magnetism. They use this information to move both locally — like a fish trying to find its way home — and over long distances. Salmon are some of the best known orienteers and put their ability to good use when undertaking long distance migrations from fresh to saltwater. Sharks are also well known for their electromagnetic sense and, for that reason, some people believe that you can repel sharks by creating a magnetic field that essentially scrambles their senses.

Even though many fish have magnetic cells in their bodies, you still won’t catch them while magnet fishing. In fact, that magnet at the end of your line might serve to repel them rather than attract them. Regardless, it’s amazing to think of these complex underwater navigation systems being created by something so simple as magnetism — and that now we can use highly sensitive tools to detect what we otherwise wouldn’t know was there.

Susan Olcott is the director of strategic partnerships at Maine Coast Fishermen’s Association.