Magnetotactic Bacteria
Several species of bacteria manufacture magnetite, naturally occurring magnetic form of iron oxide (Fe3O4). Some chemically alter their environment, precipitating this mineral from their surroundings. Others, such as Magnetospirillum magnetotacticum, internally sequester crystals of magnetite into structures (magnetosomes) that are tens to hundreds of nanometers long. They use these permanent magnets to orient themselves along magnetic fields. When left behind in sediments, their magnetite yields information concerning the orientation of the geomagneticfield in which they lived.
Magnetite crystals in the same size range as those made by Earth bacteria were recently found in martian meteorite ALH84001. Because this kind of magnetite crystals are very specific in structure and only found in living organisms, many are using this discovery as another piece of evidence supporting life on Mars.