This is the foundation from a home built in 1928. First time seeing this type of foundation. It appeared to be concrete filled tubes of some kind.
Bunker.
As you said it looks like hollow terracotta (clay) filled with concrete/cement.
Though I’ve seen a lot of terracotta block foundations, this style is a first for me.
BEWARE!!! Those seem to be Michigan Pre-historic Termite tubes!!!
Looks rock solid.
Bet that would cost a fortune today.
Michigan Termites Tubes… now that is funny.
The bigger issue is the amount of undermining / loss of footing support. There was an attempt to repair.
This house had active K&T wiring through-out and spliced to newer wiring at every turn.
John, did you grab that level while you were in there? 
Never seen anything like it.
Looks like half round shoring.