sourdough fermentation

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huge cavity in sourdough breaddoes anyone know why this happened to my sourdough loaf?

This type of sour dough is like a Ciabatta with a higher hydration value. When I make a Ciabatta, the 2nd rise is what is important.  After the 2nd rise, I flip the dough upside down on a cookie sheet with parchment paper.  I then dimple the dough with my fingers almost right through to the bottom.  This disperses the gas for a more even rise.  The gas pocket is a result of the hydration value.