Q: Why does my apple pie develop a "tunnel" whereby the apples have cooked down and the top crust puffs up?
Food Network: The usual culprit for a tunneled-looking apple pie is apples' natural tendency to break down and shrink when cooked. So your crust, which you laid over the apples with the best of intentions, is getting pushed up by the steam generated by the cooking apples, while the apples themselves are shrinking down.
There are a few things you can do to fix this. The first is to choose baking apples for your pies, ones that won't turn quite so much into applesauce upon being cooked. You could also precook your apples just enough so that the initial stages of breakdown take place, and you'll at least have a sense of where they stand in relation to the crust. You can also cut venting slits in the top crust, to ensure that any steam generated has a chance to escape, and won't force your crust to puff up.
