The Lady's Dark Sea
I tell you cutter, there is more to the Cage than most bloods, even the most canny of em, know. For instance, what if I were to tell yeh, that there was a series of huge flooded caverns down below the streets we're standing on? I'd tell you true mate; there is! It is nearly a half of a mile down but it is there and it isn't any sodding odd water either, not styx, not oceanus, not ..well not entirely polluted with all the refuse and runoff from the berks above but normal relatively clean water. There are even caverns large enough to hold an entire sea, fishes and all. …I can show you, for a price of course.
--Alric Jinkbiter just before he was laughed out of the tavern.
The Lady's Dark Sea lies deep under Sigils streets held in a maze of caverns connected to underground rivers that seem to run the full circumference of the ring. Those few who seem to know of it call it the "Dark" sea as a play on the word with both of its meanings, dark to mean with little light and the planar vernacular dark to mean secret, applying equally well to the sea. In depth, length and breadth from what I, Jamus Metrice II exploration expert working with the Sigil Chapter of the Fraternity of Order's IX-34b Mapping Initiative, have seen the water-filled caverns are truly worthy of being classified as a sea. However, like much else in Sigil the exact layout of the caverns does not seem to remain constant. Thankfully the six largest of them seem to be constant and stable; although the smaller ones I have found to be there one day and gone the next, shifting in a similar way that the buildings of upper Sigil seem to. With such conditions an exact survey of the underground area would be relatively futile. --Guvner's Report most of which has been lost.
The Lady's Dark Sea is held inside several major caverns, depending on your source of information up to six primary caverns and uncountable secondary chambers, that lie spaced evenly below Sigil's streets. Each cavern theoretically is connected with the next, but the connections are at best a maze of smaller submerged caverns, often filled with dangerous underwater flora and fauna as well as deadly strong and fast currents. These currents made many times more dangerous with the plethora of jagged, sharp corals, kelps and sea grasses that seem to thrive in the depths. If anyone has managed to navigate through the caverns for the circumference of Sigil's ring then they either died long ago or haven't told anyone. The main reason that most scholars and loreweavers believe that the caverns are connected and do in fact ring the City of Doors is the evidence given by their constant tides. In all of the major caverns the water flows predominantly from one direction to the other and the tide rises and falls on a regular cycle on the surface. The deeper portions of each cavern are more stagnant with fewer currents and different dangers all their own.
This is just a start. An idea I'm throwing out partly to look for feedback and critism. Questions to are welcome.