Furnace-rerlated holes in basement slab?

House from 1920. The basement floor appered to havbe been poured around the old gravity furnace. That’s the big patch. There were two more patches about the size of the old ducts used with those furnaces… 14"-16" or so.
I pulled the patch out of one and the soil beneath the patch had some concrete chunks mixed in with the soil.
Why would ducts go to the dirt floor? Above the one hole (located in a closet) shelving and the main floor were cut away and the cut had a radius similar to the hole in the floor, so it appeared cut to allow for routing of ductwork.

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Any chance the 14-16" holes were for the domestic hot water tank?

Actually, I found the answer. They were air returns. There were two returns and they went down into the basment and returned to the furnace through ducts under the floor.