Cavities can spread beyond the surface enamel, invading the softer dentin located directly beneath. It can also move to the nerve and blood supply of the tooth contained within the pulp, leading to abscesses and infections.  

Cavities can form on the biting surfaces of the back teeth -- the molars and premolars. In these locations they usually cause pits and grooves. These are thin areas of enamel that contain recesses. They can easily trap food and plaque. As the trapped bacteria continue to produce acid, the pits enlarge to form wider grooves and finally hole. Once worn through the enamel, they can invade the underlying dentin. 

Cavities can also form on the front teeth, or on the back (lingual) areas of the teeth, where there is a smooth surface. In a smooth surface cavity, the acid must travel through the entire thickness of the enamel. 

And as we get older, certain body change can themselves lead to tooth decay. For example, the gums can recede from the teeth, exposing the roots to plaque. The roots are covered with cementum, which is softer than enamel. This makes them more susceptible to decay, and the majority of people over age 50 have some form of tooth-root decay.

Also common in older people is decay around the edges of previous fillings. Over time these fillings can weaken, leading to cracks around the edges.