The U.S. tested nuclear weapons until the early 1990s. Since then, scientists have been using supercomputers and experiments to simulate nuclear test detonations, without detonating any nukes. But there are signs the world's nuclear powers may be readying to test again: Russia, China and the U.S. are all upgrading their nuclear test sites.

NPR science correspondent Geoff Brumfiel was among a small group of journalists allowed to tour an underground laboratory where this research happens.

Read more of science correspondent Geoff Brumfiel's reporting here.

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