network of underground tunnels about 4,800 miles away, in the northern Russian archipelago of Novaya Zemlya in the Arctic Circle, Russia conducts its own such experiments, meant to model the key chemical and physical actions that occur in the run-up to a full-blown nuclear explosion, without actually causing one. Experiments at the two sites are used by both nations to help ensure their nuclear arsenals remain viable but conducted under a blanket of secrecy.

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