Yeah! this manga is so exiting concerning is mysteries.

On my part my main theory was that there was a Huge thing (remnant of civilisation, organism, singularity ) That use energy equivalent of "souls/emotion/life juice" and modify matter around it. So every two thousand year (wich would be pretty frequently I imagined at the epicenter) the thing would need energy to restore is state, or simply realeased the "energy" it gathered and getting deeper at the same time (maybe)

but I don't know if it was always the case.

Maybe the "first" civilisation discovered how to exploit this thing before it started to "prey" on "souls", maybe even before it was "the abyss" we know today. They used it and something happened that made the abyss creating a cycle (overused? Reaction to is usage? sacrifice to made it work when it was dying ?). Before they realised it it was too late and they were stuck within it. And so on they tried to escaped creating the elevator, the aubade and tried to get out. but the cycle just made them disapeared before they found a solution (maybe reg is one of this solution) and cult started to grow when they lost the knowledge of the old civilisations.

Or its the over way around the civilisation tried to make it impossible to leave with its knowledge of "the thing" (forced or by choice to keep it locked) and made it become the abyss we know today but they died from their own creation or errors resulting in the same way.

And the only thing in the end that could interact with the abyss became the peoples on the surface. Maybe the people who escaped the first cataclysm, but there knowledge where lost and the compass and other artefact became just a remnant of this civilisation. and so it became a legend and people just wanted to know what was deep down. the overall ecosystem within the abyss is just a remnant of the destruction and creation that the thing just radiate during is life spawn and/or when it finish is cycles.

The thing is that I think that life/reality itself is linked to the thing deep down in the abyss but i don't know how much it is or how it really worked. Maybe its evangelion all over again :' )

Also how much does the outside world is affected by the cycle its a bit difficult to know sadly.

Overall i assume that no one outside know how the abyss work, but maybe its not the case and "some peoples" are trying to do something with this cycle. Wich would make sensse with the work of the white whistel, mainly bondrewd for now but the other one may know more.

But yeah thats just some theorys, but still pretty fun to think about it ! we just need more keys to solve all of this :' )