I think you just had a large bubble of air trapped in the dough there. Did you take a rectangle of dough and roll it into the loaf shape? Most of the time when I get a bubble out like that, there is a big void/air bubble in the dough from trapped air.These just came out of the oven:
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These are the first of a few days of baking (I need to get more flour first). I'm trying to get the freezer stocked up, and enough baked to give some to my grandparents.
Am I correct in assuming the one at the bottom of the picture bubbled out because my cuts didn't start far enough down?