Dark lines on teeth are not necessarily cavities, but may be fossa staining.
When you find black lines on your teeth, you can go to the dental clinic for examination, identify the disease and then treat the symptoms. If the black lines on the teeth are simply fossa staining, you can observe them for the time being and maintain your oral health by brushing your teeth regularly, at least twice a day, flossing your teeth and having regular checkups.
If the black line on the tooth can be hooked with a probe, it is evidence of caries and needs to be treated with filling surgery. Firstly, the decayed part of the tooth should be cleaned up, and then the suitable material should be used for filling, if the decay is deep, a pulp-protecting material can be used to protect the pulp before filling the tooth.