As far as I know, "Downside" is mentioned only in a single storylet, https://fallenlondon.fandom.com/wiki/Letters_from_Downside. Is there any other mention of what is implied to be the even deeper reaches of the Neath?
Is that Flute Street and the stuff down there? The Rubberies, the stone pigs?
The linked text specifically says it’s talking about something below even the rubbery cities. I don’t know what’s down there.
there’s a sidebar snippet which says what’s underneath the city, given we know that the stonepigs and the rubbery hatcheries are there but we never hear of the masters summer homes, perhaps they have voluminous caverns in the depths of the neath to kick their feet up
Given the sidebar snippets and outlandish articles are… not exactly reliable… these seem like clear hints to the existence of Flute Street and its layers of past cities.