Dagon's Maw, or the Pit of Hatred
Some SPOILERS follow for the Tammeraut's Fate adventure (in 3.5e version and also a part of the 5e Ghosts of Saltmarsh book).
The module Tammeraut's Fate from Dungeon Magazine #106, January 2004, features an underwater portal to the Abyss. The portal is somewhat protected, but creates some evil effects nearby. Currently it would be difficult for demons to reach most of the material plane from it, but it could be possible to remove the protections. From the module, bold emphasis mine:
Although this adventure is set in the World of Greyhawk on a small island off the coast of Nyrond in Relmor Bay, it can easily be adapted....
(Anna Meyer's Greyhawk maps show the adventure's town of Uskarn and Firewatch Island in Nyrond's Viscounty of Eventide, detailed map section 38.)
When the Tammeraut broke up and sank, the battered ship settled above an underwater chasm known as Dagon's Maw. This evil chasm once held a portal to an oceanic layer of the Abyss that had long ago been sealed and warded by the shamans of several tribes of sea elves native to Relmor Bay. The shattered wreckage of the Tammeraut landed atop this seal and damaged it.
At the bottom of the underwater cavern,
The funnel-shaped hollow to the north is 3 feet in diameter and descends 2 feet to a small vent. This opening is 1 inch in diameter .... The tiny opening itself descends for hundreds of feet without widening into the depths below. The forgotten architects of this opening left it as a pressure valve of sorts to allow the evil within to slowly seep out rather than accumulate and gain power over time. The elves further sealed the opening by enclosing it below a magically warded cover. The sinking Tammeraut broke these wards, and the slumbering evil is once again spreading.
The rift above the portal is not entirely unknown to surfacers:
A character who makes a successful bardic knowledge or Knowledge (geography) check (DC 30) also knows the location of the rift.
All this could be read as saying the actual portal no longer exists, but its side effects remain. But given the efforts made by the shamans intending to endure a long time, and the effects occurring since the magical seal was broken, I think it's easier to infer that the shamans were not able to actually close the portal, only restrict its access and effects, and the actual portal below the remaining physical barriers is still open.
The 5e source book Ghosts of Saltmarsh adapts Tammeraut's Fate as its Chapter 7, and still includes the portal to the Abyss. The details of the portal are very similar, except:
- Dagon's Maw is renamed to the Pit of Hatred.
- The exact location is not specified. Using the chapter within a longer Saltmarsh campaign suggests the rift might be in the Azure Sea closer to Keoland. On the other hand, Anna Meyer's maps put most of the Ghosts of Saltmarsh locations in or near Keoland, but leave Firewatch Island, and therefore the nearby rift, near Nyrond.
- The history and nature of the "rift to the Abyss" are not described in much detail at all. The chapter does say the PCs likely "discover the damaged magical seal that once kept the portal inside the Pit of Hatred closed". It also includes an "ancient warning" written in Elvish.