Underground lines are subject to animals burrowing through them, backhoes digging them up, etc. If the power is transmitted AC, then the insulation gets heated

Underground lines are subject to animals burrowing through them, backhoes digging them up, etc. If the power is transmitted AC, then the insulation gets heated up as the alternating current interacts with the surrounding ground. If the ground is below the water table, the losses get high. The underground power cables in our neighborhood are in wet ground, and recently had to be dug up because the insulation failed. In Norway, they use DC to transmit power out to oil platforms as it does not have this problem.