|Into the most inaccessible part of the basement|
Turns out the digger dudes dug something up when they dug my hole: an old water well. It looks like the well for the house used to be ~5' away from the house right beside where the hole is. The digger dudes unearthed the well casing, which they had to expose and my contractor had to cut off.
Now, given that our well and hot water tank are on the opposite side of the house, it would have made sense to put the pressure tank over there, too. But now it makes sense. The house was plumbed from the old well we unearthed, and rather than re-plumb the water supply when the new well was drilled, it was easier to bring the water to the pressure tank's existing location -- also explaining the rather unique route the water pipe runs to the pressure tank.
Surprised? In this house, never.