Magnetic dip is the angle between the horizontal and Earth’s magnetic field. A compass needle, for example, will not point north and south, but will also have a dip. It tends to dip at an angle toward the Earth (and to the sky). The dip is generally greater toward the pole. At various locations close to the equator (but not exactly at the equator), the dip is zero.
Magnetic dip as observed on various locations on Earth can only happen if the Earth is spherical.
If we are close to one of the poles, the magnetic dip will pull compass needles toward the ground. This makes a compass practically unusable near the poles.
Flat-Earthers read stories about Admiral Byrd and how his compass didn’t work when he is in Antarctica. They would jump to the conclusion and assume the compass didn’t work because the Earth is flat. They are wrong. The compass didn’t work because of magnetic dip, and the same phenomenon happens not only in Antarctica but near the north pole as well.
At some locations near the equator, magnetic dip is zero. Earth’s magnetic field would pull compass needles perfectly horizontal. This happens because Earth’s south pole is affecting the compass needle’s north pole with the same force Earth’s north pole is affecting the needle’s south pole.
Again, some flat-Earthers hypothesized that because the Earth is spherical, then a compass needle should point toward the ground at an angle. But we don’t witness this, and they concluded the Earth is flat. They are wrong, again. This misconception arises from their assumption that the needle is attracted to the north, but they appear to be ignorant of the fact that the other point of the needle is also drawn to the south. All magnets —Earth’s included— have two poles. There’s no such thing as a monopolar magnet.
Most observations concerning Earth’s magnetism cannot be explained in the flat Earth model. On the other hand, everything is consistent under the globe model.