Annie - from Vallejo, CA
Underbaked cakes will dip in the center as they cool.
Make sure the temperature in the oven is correct - as an oven gets older, thermostats can be off. Check with an oven thermometer by placing the thermometer on a shelf in the center of the oven - centering it on the shelf. Allow it to sit in the heated oven 10 minutes before checking. If it's incorrect, you can adjust the thermostate setting or call a repairman.
If you oven is correct, check the baked item with a cake tester or toothpick before removing from the oven. Test in the center of the cake layer - the tester should come out of the cake dry or with just a few moist crumbs.