The Bore was created, where the thinness was; it was drilled through that thinness to reach the DO (as said, the DO probably thinned the Pattern at one point with the TP before):

As Demandred notes:
"Here [in the Pit] he could sense the Bore, the hole drilled through so long ago to where the Great Lord had lain imprisoned since the moment of Creation. Here the Great Lord's presence washed over him. Physically, this place was no closer to the Bore than any other in the world, but here there was a thinness in the Pattern that allowed it to be sensed."

You guys should probably also re-read Herid Fel's discussions about the Bore's nature with Rand in LoC and remember that Fel was killed because someone thought he knew too much.