How Do You Keep Dogs From Digging Out From Under A Fence?

Rik is having trouble with his two adorable pooches, Barkley and Phoebe. He recently had a fence put up around his yard to keep them safe and secure, but every day when he gets home they are both out running amok through the neighborhood. OK ... they aren't running amok, but they are getting out every day.

The problem is that they dig under the fence and slip out.

Rik needs help trying to figure out how to secure the bottom of the fence to keep them from escaping.

What's a good way to keep dogs from digging out of a fence?