It's got a fair bit of pressure bubbling up, but I can plug it up, temporarily at least. I've tried as such for a day or two and nothing in particular has happened... there are one or two cracks elsewhere where water gets through, but at a much slower rate. I could just tank the floor and forget about it, but I'm not sure what that'd do to my foundations long term, or if that'd just rise the water table further and it'd start coming through the walls... Though I can't imagine I'll stay in this house longer than a few years or so...
As for why it was drilled, maybe someone wanted to check the depth of the concrete, maybe they were trying to ease the water pressure or let the water flow... I've no idea... Feels like something the previous owner should have disclosed... No way he didn't know the issue existed when I bought the place...