The mass production of silver coins in the Roman Empire had an unintended effect: spewing so much lead into the atmosphere that it may have lowered citizens’ IQs.

It was during the period known as the Pax Romana, a time of stability in the Roman Empire that lasted for about two hundred years between 27 BC and 192 AD. Mining and smelting operations throughout Europe were likely a a source of lead contamination,

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"A 2 to 3 points lower IQ doesn't sound like much, but when you apply that to the entire European population, it's quite a lot."

Two thousand years ago, the Roman Empire hummed with activity. Roads stretched like arteries, connecting distant provinces. Mines yielded treasures buried deep in the earth, and smelters worked tirelessly to transform raw silver ore into the coins that fueled an empire.