Armour loss in underground cables
Sheathed cables running AC have inductive coupling to the sheath and armor, which induce eddy currents leading to parasitic losses. More here: https://mycableengineering.com/knowledge-base/cable-sheath-and-armour-loss.

The fill also absorbs some of this coupling resulting in further losses.

High-voltage DC (HVDC) was developed to counteract these losses. HVDC avoids eddy currents in the sheath/armor and capacitive losses, as well as skin effect in the cable.