Scientists found a few years ago that migrating fish, like Salmon, that unerringly return to the same river year after year to mate, have deposits of an iron-rich compound at the front of their head. These iron crystals pick up the Earth's magnetic field, almost like an internal compass, allowing the fish to navigate. Now scientists have found that people too may be sensitive to the Earth's magnetic field, possibly explaining how birds and other migratory animals know where to go. Unlike Salmon we don't have iron deposits in our heads, but it is thought instead that we might be able to unconsciously 'see' Earth's magnetic field, helping us to navigate. To test this idea a team in Leipzig, Germany, led by Dr. Franz Thoss, measured the lowest level of light that volunteers could detect in a spot shone straight ahead of them. They found that when they shone the light at the same angle as the Earth's magnetic field, in other words so the light and the magnetic field followed the same path, the volunteers could detect a much dimmer light than when they were facing in other directions. The effect was very small, but nonetheless significant, say the researchers, who think that the magnetic field alters the way the light-sensitive (photo) receptors in the retina work so they become slightly more sensitive when lined up with the Earth's magnetic field [J. Comp. Physiol. 188: 235].