Every 500,000 years or so, the Earth's magnetic field flips upside down. The poles - north and south - do a giant somersault. It may be a few thousand years from now, but we are heading for one mighty tumble.
Considering that modern humans have only existed for 200,000 years, we have yet to navigate through a magnetic field reversal of such overwhelming magnitude. Compasses will confuse themselves; likewise birds, bees and bacteria may spin in random directions.
The Earth's magnetic field also stretches several hundred miles into outer space. It protects us from the sun's charged particles and cosmic rays by focusing them towards the poles, creating such wonderment as the northern (and southern) lights. But as it turns in upon itself, it will start to weaken. It could even temporarily disappear.
In geophysical terms, this temporary lapse easily spans a human lifetime. We are talking 100 years or more. With no protective magnetic shield, weather patterns such as wind factor, atmospheric pressure and temperature are likely to be drastically affected. We could feel the full heat of cosmic radiation - and eventually, genetic mutation.
Closer to the ground, as north swings south, continents might lurch this way and that, triggering earthquakes as they tear themselves apart. Pretty disruptive stuff. Well, potentially.
According to fossil records (sediment cores taken from the ocean floor) hundreds of pole reversals have occurred throughout the timeline of our planet; the last one happened during the Stone Age. Back then, the records showed no drastic changes in plant or animal life. Back then, it was a very different climate.
Depending on who you talk to, the next flip could prove pretty interesting. One theory is that the magnetic field will become more complex - instead of having just one north and south pole, we may get two (or more) of each. These could wander around for several hundred years before settling into a new arrangement.
Whatever the scenario, it's comforting at least to realise that these reversals don't occur overnight. They take anywhere from 1,000 to 10,000 years. So even if we in the middle of a such a geological event, they are so damn slow that it's hard to tell if anything is happening at all.