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St. Robert Bellarmine, On Purgatory, Book 2, Chap. 6: “The eighth is the common position of the scholastics: that Purgatory is within the bowels of the earth, near to Hell itself.  The scholastics by a common consensus designate within the earth four cavities, or one divided into four parts: one for the damned; another for those to be purged; the third for infants dying without baptism; the fourth for the just who died before the passion of Christ, which now remains empty.”

The Dimonds posts this quote recently, I thought it was quite interesting. I have never considered purgatory also being in the earth.

So, it's interesting, but apart from pure speculation, I'd like to see if they have some actual reasons for saying this.  Perhaps something the Church Fathers said?  Now, one problem with that is the notion of Limbo of Infants wasn't developed until the 12th century, and at the same, some rejected the notion as Pelagian.  So, then, if the concept had just been devleoped in the 12th century, how could they know where it is?

I personally believe that those in Limbo will end up in some place that's adjacent to where the Kingdom of Heaven proper would be, and that they would interract with those who have been saved.  Of course, that would be then, when they all have bodies after final resurrection.

Now, I do think there's consensus regarding the location of Hell being in the earth, and also the Limbo of the Fathers, since otherwise Our Lord "descending" to the "inferos" (lower regions) would make no sense.

I also don't know what the rational for putting Purgatory down there would be.

Hell, yes.  Limbo of the Fathers, yes.  But for Purgatory and the Limbo of Infants ... I think we simply don't know and it's pure speculation.

IF the Limbo of Infants is in the nether regions, furthermore, what's to say they could not be inhabiting the place where the Fathers used to be before they had been liberated, and that the place is currently empty?  I don't think that part follows either.

As I understand it, there's no suffering in Heaven, and there's nothing up there apart from it. So, as Purgatory is a sort of temporary Hell, it makes sense for it to be down, and not up. The story of Lazarus also seems to imply that the Limbo of the Patriarchs and Hell are close to each other.