This fungus is causing a heart rot on the interior of the tree.  This fungus has been growing for many years in the tree.  It started when the tree was wounded sometime in the past.  Such wounds can be as serious as a fire scare, some traumatic mechanical damage, or as trivial as a poorly closed broken branch scar. There is little that can be done for the problem, other than keep the tree healthy and free of additional stresses.  However, it is critical to know that the structural integrity of the tree may be comprimised, posing a danger for property or humans.  If the tree poses such a danger, then I recommend a knowledgeable person be contacted to inspect the tree and insure it will not cause property damage or bodily harm should it fail. I am not sure of the species of the fungus, but it looks like it may be a young Ganoderma fruiting body.