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The United States has provided more information supporting its claim that China secretly conducted an underground nuclear test in 2020.

Some shit you should know before you dig in: Two weeks ago, the US formally accused China of conducting a secret nuclear test on June 22, 2020, marking the first time the US publicly identified a specific date tied to such an allegation. The claim was made by Thomas DiNanno, US Under Secretary of State for Arms Control and International Security, during remarks at a UN disarmament conference in Geneva. DiNanno said that China had carried out low-yield nuclear tests with designated yields in the “hundreds of tons” and had used seismic masking techniques like “decoupling” to conceal the activity from international monitoring systems. Despite his claims, Chinese officials have denied the allegations, calling them baseless and politically motivated.

What’s going on now: While speaking at the Hudson Institute, Assistant Secretary of State Christopher Yeaw provided more detailed evidence to back the administration’s claim that China conducted a covert underground nuclear test in 2020. He said a seismic station in Kazakhstan detected a 2.75 to 2.76 magnitude event about 450 miles from China’s Lop Nur test site and argued that the signal had “very little possibility” that it was anything other than a single explosion. According to Yeaw, the seismic wave patterns were inconsistent with earthquakes or mining activity and aligned with what would be expected from a nuclear explosive test.

Yeaw said he had personally reviewed additional data and concluded the blast was at least “supercritical,” though he acknowledged it was impossible to determine the exact yield. He reiterated claims that China likely used “decoupling” techniques, detonating a device inside a large underground cavity, to suppress the seismic signature and conceal the true magnitude of the explosion. He also pointed to prior indications that China had been preparing tests in the “hundreds of tons” range, suggesting the 2020 event fit within that broader pattern of activity.

Despite all of this, the evidence remains disputed. The Comprehensive Nuclear-Test-Ban Treaty Organization reported detecting two very small seismic events 12 seconds apart but said the data alone was insufficient to confidently assess their cause. Independent experts have also warned that the signal was weak and recorded at a single station, meaning it cannot conclusively rule out natural seismic activity or non-nuclear explosions. Yeaw did not disclose whether the US possesses additional classified intelligence, such as radioactive detection, human intelligence, or satellite evidence, to back his claim.

This all comes as President Trump has said the United States would resume nuclear testing “on an equal basis” if China or Russia conducted nuclear tests.