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

Chinese researchers have showed a handheld quantum magnetometer built for hunting submarines in a striking real-world test: it tracked a subway train rumbling beneath a city street.

The device, developed by Peng Xinhua and colleagues at the University of Science and Technology of China, is described in a study published on September 14 in Acta Physica Sinica.

Though it is scarcely larger than a pencil and draws only 5 watts of power, it can isolate the magnetic disturbance caused by a moving train in the Earth’s vast background field, clearly recording the train braking into a station, its stop and its departure under traction.