The Earth's magnetic field, which protects us from potentially dangerous solar radiation, is gradually losing its stability. No need to move underground or build space colonies just yet, though: the changes are taking place over millions of years.
You might assume that compasses will always point north, but in fact the magnetic poles have swapped places many times in the Earth's history. Earth scientists have long suspected that these flips are becoming more frequent, and that the magnetic field was less prone to pole reversals in the distant past.