These lines are called galleries, and are created by the larvae of the European elm bark beetle under the bark of a dying tree. According to Charley Eiseman, the centre, deeply grooved line is the spot where the parent snuggled in, and the lines radiating outward are made by each of the many larvae. To me, these galleries are attractive and look like a work of art. This was one of many on an old tree that had long since fallen down and lost all its bark.