It looks like there was a large bubble in the dough before it went into the oven.
Can you say more about your recipe/method? If you are using the recipe described here, https://www.youtube.com/watch?v=2b5jiHZ4uK0 then I would suggest looking at the dough before it goes into an oven, and using a skewer of fork to prick any obvious large bubbles.
[/url]
It looks like there was a large bubble in the dough before it went into the oven.
Can you say more about your recipe/method? If you are using the recipe described here, https://www.youtube.com/watch?v=2b5jiHZ4uK0 then I would suggest looking at the dough before it goes into an oven, and using a skewer of fork to prick any obvious large bubbles.