Imagine you're packing for the trip of a lifetime: Antarctica! You're going to see the South Pole and maybe hang out with some penguins. But how are you going to find the actual South Pole?

You might think that you would reach for a compass first. Compasses — navigation instruments that contain magnetized pointers — have helped people find their way around Earth for thousands of years. The planet's magnetic field attracts one end of the compass's magnetic pointer toward the North Pole, so compass users always know which way magnetic north lies. 

However, you may be surprised to learn that compasses behave strangely when they are close to the South Pole. Why is that?

Earth has both geographic and magnetic poles. The geographic North and South Poles mark the opposite ends of a central axis that Earth spins on. However, the positions of the North and South magnetic poles aren't fixed points, and their distance from the corresponding geographic poles can vary by as much as several thousand kilometers. 

Earth's magnetic field is generated by the spin of the planet and the sloshing of iron-rich fluid in Earth's core, so the magnetic field — and the magnetic poles — shift in response to the speed and pattern of the fluid’s movement. 

Compass needles are designed to align with Earth's magnetic field, with the north end of the needle pointing to the magnetic North Pole and the opposite end of the needle pointing to the magnetic South Pole. When you take out your compass and let the needle settle, it will run parallel to the lines of Earth's magnetic field where you are standing.

But Earth's magnetic field is not arranged in straight lines all the way from the North Pole to the South Pole. As you get closer to the magnetic South Pole, the field lines will curve to dive straight into the magnetic South Pole, running perpendicular to Earth's surface. "So quite often, compasses actually won't work," said Tom Jordan, a geophysicist with the British Antarctic Survey. "Instead of trying to point horizontally, what your compass needle's actually trying to do is point straight down into the Earth."

What that means is, if you were to visit the South Pole bearing a compass with a free-floating needle that could move in three dimensions, the "south" end of that compass needle would point straight down once you reach the magnetic South Pole, Jordan told Live Science.

That compass would behave similarly at the magnetic North Pole; the north end of the needle would be trying to point straight down into the ground, according to Jordan. 

Because compasses behave oddly near the magnetic North and South Poles, polar explorers used to calculate the location of due north by mapping the angle of the sun or the positions of stars, Jordan explained. Today, people trekking around Antarctica use GPS to figure out which way is north. What they do is move around a bit to figure out which way is north, kind of like when you don't trust that your phone's maps app knows where you actually are and which way you are actually facing.

It's only on the equator that a typical compass will provide the most accurate reading about which direction is north and which direction is south, Jordan said. That's because at the equator, all of the planet's magnetic field lines are horizontal and parallel to Earth's surface, he explained.

• Originally published on Live Science.