I have a cavity under and visible below a crown. Can the tooth be saved?

Unfortunately, any place you have had dental work done, that treatment (whether it is a filling, a crown or a veneer) always connects to the remaining tooth. As long as you have tooth structure you can get another cavity there.

If the cavity you see is very small- like a pin point, then it sometimes can just be filled. Sometimes that cavity actually goes under the crown, and in that case the crown has to come off, the cavity removed, and a new crown made.

If the cavity you see is larger than a pin point that that can be a problem. Often what you see is just the tip of the iceberg. 

Go to your dentist right away. Be prepared to have a new crown made. Hopefully, it is still repairable.

(After it is fixed, make sure your dentist discusses how to prevent these types of cavities from ever coming back).

Yes.  The tooth can be saved and usually is salvaged by replacing it with a new crown.  However, removing a crown is like lifting up a rock in the woods (or the desert here in Las Vegas).  You really don't know what you will find until it is removed.  At that time your dentist can remove any decay and determine if the tooth is salvageable.  Until then, we are just guessing.

It depends. When there is decay under an existing crown it is impossible to tell how extensive the decay is until the crown is removed. X-rays can't detect decay under a crown so until the crown comes off it is impossible to know how extensive the damage from decay is. I tell my patients it is a lot like pulling up carpet in an old house. You never know if there is a beautiful hardwood floor or six layers of linoleum. The sooner you get the crown off to evaluate the situation the better. I will say that the majority of the time the tooth can be saved so the odds are in your favor. 

Good luck!

You need to get evaluation with your general dentist to make informed decision. It seemed like a defective crown margin as per your question which will be difficult to save. X-ray and photos will be helpful.  

It happens very often when there is an extensive overhangs, opened/closed contact, poor hygiene. The depth and extensity of the decay lesion can be partially defined with x-ray. However, most often it is quite complicated to remove all the decay. Your dentist will define if the crown should be removed and changed.