We made a survey along the tubes in the last 1km arm reaching the terminal buildings. To be reminded, these tunnels are powered with the NEB and WEB IPS mains line, likewise the first slice in the beginning is powered by the CEB IPS, while the intermediate tunnels are conected to the 1500N and 1500W IPS mains line.

We probably found the device generating the +/- 6Hz sidebands of the mains in the WEB magnetometers: it could be a small (2 kVA) UPS installed on December 28, 2023 at 2100m west arm (900m from WEB). This UPS is only connected to the " T " IPS line, unlike other UPSs (2400m) which are three-phase and produce the well known 13 Hz bump with its associated 50 Hz sidebands.

In the attached figure, the top row shows the magnetometers in the experimental room, while the bottom row shows the small triaxial magnetometers near the towers (vacuum chambers). Signals of a large (~8 kVA) three-phase UPS at NEB (blue trace), and those of the small single-phase 2 kVA UPS at WEB (red trace) are clearly visible in the upper left pane.

We will schedule a shutdown to verify this hypothesis.