Saga of mysterious 15 Hz ground motion at LIGO Livingston Observatory

Since at least February 2015, a mysterious noise at approximately 15 Hz has been present at LIGO Livingston Observatory's Corner Station. This feature is visible in seismometers, accelerometers, microphones, and magnetometers distributed throughout the building and can also be seen in the gravitational-wave data. The frequency of the feature can change by small (< 0.05 Hz) and large (> 1 Hz) amounts both suddenly and gradually. Various attempts in 2015 and recently to identify the source of the noise, by turning fans and motors on and off to see if it disappears, have been so far unsuccessful. Details of the feature and attempts to find the source will be described.