Look what I found in my bread this morning. This is a classic shaping issue. The are three main culprits for this.

1. Oil from the tub I have proved it in has got into the main bulk of the dough.

2. The most likely is that it was raw flour from shaping which when it hits the oven explodes causing this cavern.

3. There was a big bubble which I didn’t adequately squish out whilst shaping.