I'm planning to create an adventure where the party is trapped in Nessus, the ninth layer of the Nine Hells, and I must make my way through the layers of the Nine Hells to Avernus, the first layer, from which you can find a Portal and finally escape.
So far, all I have found with respect to the trip between layers is this (DMG, page 64):
To reach the deepest layer of the Nine Hells, one must descend through the eight layers above it, in order. The fastest way to do it is the river Styx, which sinks more and more as it flows from one layer to the next.. Only the bravest adventurers can endure the torment and horror of that journey.
There are no flat portals that connect directly to the lower layers of the Nine Hells., by order of Asmodeus. As such, The first layer of Avernus is the point of arrival of the visitors to the plane..
The River Styx, then, seems to connect all the layers, but I did not want the party to simply navigate, go through all the layers without actually experimenting and surviving each, so I'd rather not use the Styx River if possible.
Are there other ways to travel from one layer of the Nine Hells to another?, ideally in descending order (ie from 9 to 1)?
This will be for D & D 5e, but information from previous editions is welcome. If the Hells differs between the editions, assume the Forgotten Realms (although the information of other configurations will not be rejected because they are not Forgotten Realms). I also consider that the level of the party or what I plan to place in each layer of hell for the party to occupy are beyond the scope of the question, although I will point out that none of them will have access to change of plane or similar.
As an example of the kind of things I'm looking for, the Wikipedia page in Baator says this about Cania:
Many things hide under the ice of Cania: vast lost cities, frozen armies of creatures. The main portal to access Nessus is through a huge hole. Protected by 9,999 gelugons. At the bottom of the vertiginously deep pit there is a body of icy water and 1,001 fathoms deep. a silver portal to Malsheem.
However, there are no sources I can see for this paragraph, so I do not know if any of this is true. But at least it implies that there are portals that connect one layer to another …