Evidence of that decline has come from a surprising source. People have been making pottery for thousands of years. Archaeologists study pots to learn about ancient cultures. But these vessels have another story to tell.
JOHN SHAW: Pottery acts just like a magnetic tape recorder. It records the Earth's magnetic field when the pottery is first made.
NARRATOR: An ancient pot is a magnetic time capsule. John Shaw has learned how to extract from it a precise measurement of the strength of the magnetic field as it was in antiquity.
Like volcanic rock, clay contains tiny pieces of an iron-based mineral called magnetite. At the microscopic level, magnetite contains lots of distinct magnetic regions, in effect, tiny magnets. But in raw clay, these microscopic magnets all point in different directions, so they fail to create an overall magnetic field. That means a lump of clay on the potter's wheel is not, itself, magnetic. Not yet, anyway.
JOHN SHAW: Now the interesting part is when the pot's fired.
NARRATOR: The intense heat in the kiln erases all the magnetic regions. But as the pot begins to cool, new magnetic regions form in the magnetite. And as the regions reform, they align with the Earth's magnetic field, just like compass needles. With millions of tiny magnets all pointing in the same general direction, the pot itself becomes slightly magnetic. Once it has cooled, the magnetism is locked in.
JOHN SHAW: So if we take an ancient pot like this one, which is from Peru, when it cooled for the first time, it cooled in the Earth's ancient magnetic field and it became magnetized in that field. And of course, if the field's very strong, then the pot's strongly magnetized, and if the field's very weak, then the pot's weakly magnetized.