Within a tiny salmon egg, molecules of magnetite are already gathering.

Once the egg hatches and the salmon begins to grow, it moves downriver, eating larger prey, taking in iron from its environment and its food, to synthesize more magnetite to store in its body—particularly in the sinus bones within its skull.

Once it’s large enough to enter the ocean, its scales change from green and brown to the silver and gray of the open sea.

The magnetite in its nose begins to line up into chains that can detect and respond to the magnetic fields of the Earth.

The salmon is developing its magnetoreception.

In the ocean, it feeds on fish and krill, ingesting more iron, storing more magnetite, traveling thousands of miles—up to 18 miles a day—over the next few years, guided in the dark waters by its three-dimensional magnetoreception, sensing not only direction but intensity and inclination of the magnetic field.

When it’s time to return to its home river, magnetoreception is aided by another sense in its nose: smell.

The salmon can detect just a few parts per million of its birth river in ocean currents and follow them home. Once there, it will mate and die.

Nutrients and minerals from its body will return to the stream to nourish future salmon, who will make their own magnetite for their own magnetoreception, to guide their own miraculous journey, handed down across thousands of generations.