Dental Amalgam is a mixture of metals, consisting of liquid mercury and a powdered alloy composed of silver, tin, and copper. It has been used in dentistry to fill cavities for more than 150 years, but because they are metal, they have no  adhesion to the tooth, time wearing and mouth temperature changes create a gap around the filling, wide enough for bacteria to get in the base of the restoration and produce cavities inside. 

That leads to a failure of the restoration, sometimes even the remaining tooth breaks down.

 Nowadays amalgams are prohibited by the Dental Association and considered toxic to the body. 

To remove it the dentist  isolates the tooth, mercury safe handling, and performs the filling substitution with  a high aesthetic composite.