This picture is a perfect example of why it is so important to catch problems when they are small.
In this tooth, decay started in between two teeth (the white arrow) and progressed to the nerve (the blue arrow). Remember, cavities don't always cause pain and many dental issues may not produce symptoms until they are advanced. Since the decay in this tooth started between two teeth, this would not be seen without the aid of an X-ray, which helps us see areas that cannot be seen upon visual examination alone, such as between teeth and under any existing restorations. This is why we take routine X-rays.
Our bodies have a central nervous system in place to warn us of things that may be wrong. However, pain is not a reliable indicator and patients often make the choice to delay treatment because there is no pain. Unfortunately, in many instances by the time you have actual symptoms, the problem is too big to treat conservatively.