China tested a handheld submarine detector. It tracked a subway train underground

Researchers at the University of Science and Technology of China, led by Peng Xinhua, have developed a handheld quantum magnetometer. In a real-world test, this device, described in a study published on September 14 in Acta Physica Sinica, successfully tracked a subway train moving underground. The compact device, powered by only 5 watts, demonstrated its ability to detect the magnetic disturbance caused by the moving train, even recording its braking, stopping, and departure. Originally designed for submarine detection, this test highlights the device's sensitivity in isolating subtle magnetic signals within the Earth's background field.