There is a large growth on the trunk of my maple tree. What is it?

FAQ
Question: 

There is a large growth on the trunk of my maple tree. What is it?

Answer: 

The growth on the trunk of the maple tree is likely a burl.  Burls are abnormal swellings or growths that develop on the trunks and branches of trees.  Burls can be found on deciduous trees and evergreens.  The exact cause is unknown.  Possible causes include bacteria, fungi, insects, wounds, or environmental stress.  

Burls do not kill trees.  However, they may reduce the tree’s vigor.  On a positive note, the unusual swirling grain pattern found in burls makes them prized by woodworkers.  Burls can be carved into bowls, furniture, and other objects.