Giant Gaping Hole

Toast

I'm fairly new to making sourdough bread and ran into this giant air bubble on my last loaf. Any ideas what may have caused this? 

I followed this recipe: https://www.theperfectloaf.com/country-sourdough-less-levain-longer-autolyse/

My guess is that you enclosed a giant bubble in the dough when you formed the loaf.  Only a guess!

Ford