What are tunnels in quick breads caused by?

Tunnels are caused by excessive moisture that builds up in the quick bread batter during baking. This moisture can come from a variety of sources, including:

* Adding too much liquid to the batter

* Not measuring ingredients correctly, particularly when measuring dry ingredients

* Using over-ripe fruit or vegetables, which contain more moisture

* Overmixing the batter, which can develop the gluten in the flour and cause it to be too elastic, resulting in tunnels when the loaf is baked

* Using a baking pan that is too large for the amount of batter, which can cause the loaf to bake too quickly and result in tunnels

* Not baking the bread for long enough or at the correct temperature