DC underground cables produce a static magnetic field. (DC overhead lines do so also, but there currently are no DC overhead lines in the UK.)
Because the magnetic field from the cable alters the earth’s magnetic field, there is an effect on magnetic compasses close to the cable – they can point in the wrong direction. This can be an issue for ships at sea but very rarely causes problems on land, partly because the effect is localised – it is significant only within about 10 m of the cable itself.
The following graph shows the absolute magnitude (ie the size, not the direction) of compass deviation for a typical cable. This is the same typical cable for which we give graphs of the field. The compass deviation depends on the direction of the cable. This graph is for a north-south cable, which produces the largest deviation.
Note that these calculations are for a typical underground cable. For subsea cables, the two cores are usually further apart, widening the zone in which magnetic fields may be significant.