When archaeologists peered inside Tutankhamun’s tomb for the first time in the early 1920s, they found antechambers packed to the brim with thousands of artifacts: statues, furniture, jewelry, clothes, chariots, paintings. Among these possessions was an iron dagger — just over one foot in length and crafted from an iron meteorite — that would puzzle researchers for nearly a century.

It’s easy to see why the researchers might be confused. The Iron Age, a period when people across Europe, Asia and Africa began making tools from iron ore through a process called smelting, is generally thought to have begun no earlier than 1200 B.C. — some 150 years after King Tut’s death. If smelting was off the table, archaeologists wondered, how might the dagger have been made?

Research quickly confirmed that they demonstrated two additional classic characteristics of the transition to a superconducting state.  The material expelled magnetic fields as superconductivity occurred, allowing a magnet placed on a chunk of the material to hover above the surface. And during the transition, their heat capacity – the amount of heat required to raise their temperature by a certain amount – displayed a notable abnormality.

But despite decades of effort with a variety of experimental tools, the fourth signature, which can be seen only on a microscopic scale, remained elusive: the way electrons pair up and condense into a sort of electron soup as the material transitions from its normal state to a superconducting state.