Underground cables are armored in order to protect them against mechanical damage. The armour layer usually comes either of two forms: (1) steel tape armour, or (2) steel wire armour. Wire armour tends to be used on cables with a larger diameter, as it makes them more flexible compared to cables with tape armour.
In both cases, the armour is spiral-wound towards the outer part of the cable along its length and is, itself, protected with a hessian or plastic outer layer.