WHAT IS THIS??

Don't forget, the block was probably NEVER flushed during its life either. It should be done every few years to clean out the crud that builds up in the cooling system. Antifreeze will turn acidic after a few years & will slowly eat away at the cooling system (Cast iron, aluminum etc.) if not changed once every couple years. You will probably find rust scale & such in that gunk too.
Thats why core plugs rot from the inside out. Not many people do this.