Q: It looks like something is eating away at our sweetgum tree. What caused this and what should we do?
A: It’s difficult to see in your photos exactly where the damage started but this kind of trunk rot is almost always associated with water entering the trunk above the point of damage. The entryway is typically a stub left when a limb breaks off but it can also be from the crotch where two competing limbs both head for the sky.
In any case, the damage to your tree trunk is substantial. The trunk is weak and it may snap in a storm.
Hire an ISA certified arborist to visit and advise on what you should do.