Earth’s core is the source of the very important but poorly understood magnetic field that surrounds the planet and extends outward. One of physics’ great mysteries, it was first recorded about 180 years ago and when it was noticed to be weakening, the potential consequences, such as its effects on the biosphere, set alarm bells ringing.

However, new evidence shows that instead of diminishing, Earth’s magnetic field is perhaps merely fluctuating, as it has done over millennia. Using a set of 67 jars, all of them from between the eighth and second centuries B.C. from the region that was known as Judea at the time, scientists have gathered information about changes in the planet’s geomagnetic field in the course of those 600 years.

A study by researchers from Tel Aviv University, the Hebrew University of Jerusalem and the University of California, San Diego, published online Monday in the journal Proceedings of the National Academy of Sciences, said: “The reconstruction of geomagnetic field behavior in periods predating direct observations with modern instrumentation is based on geological and archaeological materials and has the twin challenges of (i) the accuracy of ancient paleomagnetic estimates and (ii) the dating of the archaeological material. Here we address the latter by using a set of storage jar handles (fired clay) stamped by royal seals as part of the ancient administrative system in Judah (Jerusalem and its vicinity).”

The data they found showed the magnetic field was relatively stable and declined gradually between the sixth and second centuries B.C. but a short 30-year period in the eighth century B.C. saw a spike, in which about 27 percent of the field’s strength was lost.

“The field strength of the 8th century B.C. corroborates previous observations of our group, first published in 2009, of an unusually strong field in the early Iron Age. We call it the 'Iron Age Spike,' and it is the strongest field recorded in the last 100,000 years. This new finding puts the recent decline in the field's strength into context. Apparently, this is not a unique phenomenon — the field has often weakened and recovered over the last millennia,” Erez Ben-Yosef of TAU's Institute of Archaeology and the study’s lead investigator, said in a statement Tuesday.

Findings of the study could help us better understand not just the enigmatic magnetic field, but also the inner structure of our planet.