Why would my cake bake and end up with "holes" in it???

Air pockets or tunnels? Sometimes when we overbeat a batter, it can develop air pockets. Tunnels happen when a batter is mixed for too long.

How bad was it? If you look through cookbooks and magazines you may see numerous photos of cake slices with air pockets. Most people don't notice.

Ditto what Mimifix said. Only it is not sometimes, it is all the time. If this is scratch, then you overmixed when adding the flour. If it is a mix, I can't help you. But I'm pretty sure overmmixing a cake mix will cause the same problem.

Also, I always lightly bang my pans on the counter to get rid of any air bubbles, but if these are large its over mixing...

Besides the air holes in the cake - the 12" cake sunk in the middle. As I was reading your responses I remembered my husband was having a problem setting the oven one day. I re-set my oven and put in an oven thermometer - it was off 20 degrees low!! I got out my oven manual and reset the thermostat. Now it is at the right temp! The 10" and 8" layers came out beautiful - too late in the day to bake the 12" over. I will wait till tomorrow.
Thanks for your responses,
Beverly