"terrestrial magnetism"

The Earth’s core is thought to be composed mostly of iron (around 80%) and nickel.   This forms a planetary magnetic field which varies in strength and polarity.   From studies of prehistoric lava flows, it is believed that the polarity of the magnetic field reverses periodically, on average every 250,000 years.

Measurements suggest the strength of the magnetic field has declined by around 5% over the last 150 years.