Chen Youlin, a 54-year-old US seismologist specialising in the detection of nuclear tests, has been held in China since November 2024 after being arrested during a visit to Beijing to see family, his relatives have revealed. Chen, a Boston-based researcher born in China who became a US citizen in 2011, focused his published work on North Korea's underground nuclear tests and participated in US government-funded projects — though it remains unclear whether his research touched on China's own nuclear programme. China's foreign ministry has defended the detention as lawful, but advocacy group Global Reach suspects the arrest may be linked to Chinese authorities seeking intelligence on US seismic detection methods; Chen's family and a US senator are calling for his release, warning the case could deter academic collaboration between the two countries.