Dozens of demonstrators gathered outside California's Lawrence Livermore National Laboratory on Aug. 9, 2017.

The facility is used for nuclear weapons research.

Photo by Justin Sullivan/Getty Images.

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Do you know when the last time the United States fired a nuclear weapon?

The bombs that destroyed the Japanese cities of Hiroshima and Nagasaki are the only nuclear weapons to have ever been used against a population, but they're not the only ones America's set off. Throughout the 20th century, the United States conducted well over 1,000 nuclear weapons tests.

America set off the Trinity test — their first nuclear weapon — on July 16, 1945. But here's an interesting question: When was the most recent time they set one off?

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