The teeth and molars are not “compact”, but have an internal cavity which continues through a very slim channel inside the root through to its tip (apex). The dental nerve resides in this space together with small capillaries.

The nerve is in charge of providing a warning in the event of problems (caries, fractures or any circumstance that threatens the dental structure).
The capillaries irrigate the nerve, providing oxygen and nutrients.

Natural History of Caries (tooth decay)

Where there is caries or tooth decay, over time it deepens until it reaches the area of the nerve. At that point an inflammation or the nerve of pulp (pulpitis) begins and it starts to hurt. If not acted upon immediately (cleaning the caries and filling), and the patient only takes analgesics and antibiotics, the symptoms will improve but the caries will continue until the bacteria therein ends up necrotizing (“rotting”) the nerve. At that time it stops hurting as the only warning sign we had has been destroyed and the process of bacterial colonization begins of bacteria that lives in the absence of oxygen (anaerobic) and which over time ends up forming a very painful intradental abscess (pus in the pulp chamber) that requires immediate drainage due to the risk of dissemination.

The pain caused by a dental abscess is one of the most significant in dentistry!!!

The treatment from both the first moment of pain (irreversible pulpitis) and when it has disappeared (necrosis of the pulp) is an endodontic therapy also known as root canal treatment (commonly known as “killing the nerve” or “removing the nerve”).
Technically, it can be summarised as removing the nerve, cleaning and disinfecting the inside of the tooth and filling it with “latex” (gutta-percha) which seals it almost hermetically. 
There are molars that have 4 canals unlike the anterior teeth that only have 1, and therefore the procedure for a molar is much more laborious and complex. In addition, the entire treatment is performed indirectly as we cannot see it, as is the case with other dental treatments. In order to do so, we use electronic devices which measure the length of each of the canals (apex locators) and serial radiology.
After 2 or 3 weeks a porcelain crown should be attached to the treated molar. We thereby strengthen its structure and avoid it breaking (they become more fragile after the treatment)

Classic saying of the dentist:  "An endodontically treated tooth is a well-wrapped tooth" !!!

Let us not forget the whole chapter of Repeat endodontic treatment: old, failed endodontic treatments must be treated with the removal of the old gutta-percha, cleaning and disinfection (re-implementation) and new sealing reaching to the tip of the root (apex). It is much more complicated than new, first time endodontic treatment, but it has a success rate of over 80%.

Conclusions

1. Caries or tooth decay is painless during its first few years. This is why it must be diagnosed during annual check-ups. Its treatment is a filling.


2. The fact that it doesn’t hurt is not synonymous with the absence of tooth decay.


3. Once caries begins to hurt it can no longer be cured with a simple filling, but with endodontic or root canal treatment. In this case it will take almost triple the time and greater cost.