That is usually when they cut off the trunk from the root stock. It was also poorly dealt with by the grower. Now you can decide if you want to create a hollow, or encourage it healing over.
It's what's left of a chop. Someone cut it back to a low branch but left a stub. It died back to the base.
You may need to cut or carve to get it out or it may come out like a loose tooth. Try wiggling it. If it moves,wiggle until it comes out.
You may wish to encourage growing over.
Elm wood rots easily.
We have one here that has a hollow heart where the top sealed
itself off, but the base at the root section is hollow going up
the tree.
Good Day
Anthony
With all due respect, on closer look I can be positive it is really a chopstick !
It seems (and I could be very wrong here) it is some sort of graft and the chopstick was used to press the contact between the two plants.
I'll try the carving first.