An obvious magnetic disturbance has been repeatedly observed by magnetometers located outside of urban areas in Yangmingshan National Park and has affected the monitoring of volcanic activity in the northern region of Taiwan. The disturbance has amplitudes of approximately 3–5 nanoteslas, and regularly appears for approximately 21 hours per day, except during the New Year period, when it is observed continuously for 45 hours. This change coincides with the annual extended operation of the Mass Rapid Transit (MRT) system. A 750-volt direct current drives the MRT trains and leaks to the ground, with a maximum value of approximately 1000 amperes measured along the routes. The disturbance is dominated by the unbalanced supply and return currents resulting from the leakage. The daily changes in the magnetic noise levels are correlated with the electricity consumption of the MRT system because the leakage is proportional to the use of electric power.