When you go to the dentist and you have an examination the usual routine is to have some X-Rays taken to look for abnormalities not evident by simply looking in your mouth. But the visual exam is also very important. Back teeth, especially the molars, have grooves that are vulnerable to decay.
And X-rays will not reveal the problem until it is quite advanced. So catching the problem early is best.
Below is a photo of two molars with what appear to be stained grooves. But when these grooves are opened up by the dentist with a dental drill( or with a laser or air abrasion, less common ) the decay, i.e.tooth infection is quite apparent in the second photo.
The dilemma to drill or not to drill, comes down to judgement and experience, in my opinion.
Dr David Saraga 
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