The Undercity (Torrune)

The Undercity is the popular term for the intricate system of underground passages and chambers that extends beneath much of the city of Torrune. Its various parts have diverse origins. The oldest parts exist simply because the riverbed limestone underlying the city is naturally riddled with caves and fissures, many of which once opened out onto the riverbank (before it was built up as it is today) or into small sinkholes scattered across the surrounding bottomland. Over the millennia of the city's history, a labyrinth of sewers, cellars, tunnels, catacombs, and other man-made subterranean spaces have also grown beneath it. These are dug around and between the pre-existing natural caves, often interlinked with them, or making use of them by enlarging and opening out their typically sinuous, irregular shapes.
The oldest manmade parts of the Undercity are probably the remnants of ancient quarries. Throughout Torrune's millennia-long history, the limestone itself has been heavily mined to supply building material for the city's growth. In order to reach the stone without digging away huge amounts of overlying earth, the quarriers took to sinking vertical shafts down into the desirable layers and then excavating horizontal galleries outwards from these. Where they could ease their labors by following and enlarging naturally occurring sinkholes or fissures, they did so, meaning the old quarry network is extensively intertwined with the natural cave network. 
In latter years, some parts of these old quarry shafts and tunnels have been reused as ossuaries and catacombs for the reinterment of bones from overcrowded burial grounds at various places around the city. Other parts have been repurposed as storage cellars, often wine cellars, for buildings subsequently constructed above them. The lower parts of the old quarry excavations, like the deeper reaches of the natural cave network, are often flooded due to the slow seepage of groundwater through the porous limestone over the centuries. However, not all the quarry works lie below the general street level; many were dug into the sides of the city's hills, especially the large Syrennian Hill in the Low City's western half. These are especially likely to be reused either as ossuaries or cellars, since they are often more accessible and not prone to filling with water over time. 
The sewer network of Torrune makes up another substantial part of the Undercity. Overall it is much more recent than the quarries, since most of the city's current sewers were constructed in the last few centuries. Nevertheless there are also sections of sewer (often now unused, bricked off or partially filled with earth or rubbish) that date back further, some to the Old Empire days, some even older; these are remnants of previous sewer networks, especially under what is now the southern half of the Old City, and are often unconnected to the larger system. The sewers also are also closer to the surface than the old quarry works, since the major sewer tunnels typically run just under street level and many of them are former open-air drainage ditches or sections of old canal that were covered over to control the stink. Older sewer tunnels may lie deeper, however, due to the slow building-up of the city over time and the fact that what was street level a thousand years ago may now lie several yards below the current surface. 
This slow rise in Torrune's street level, as new construction is layered upon the detritus of previous centuries, accounts for another major portion of the Undercity: Not just the disused sewer tunnels, but the built-over remnants of old cellars, cisterns, stairwells, and undercrofts, sunken alleys turned into basement passageways by the growth of tall houses around them, whole courtyards and side-streets covered by the vaulted foundations of new edifices being raised above, half-demolished ruins filled in with rubble and paved over as abandoned quarters were renewed after long desertion.
In truth, the whole Undercity is divided (like the sewer network that makes up a large part of it) into two separate portions by the River Helvander. One is the portion that lies beneath Torrune's High City, and the other, larger, portion lies below the correspondingly larger Low City. The latter portion is not just bigger, but is also more complex and multi-layered, since the Low City has a more extensive history of all the activities that have created the Undercity over the centuries: Limestone quarrying, the periodic destruction and reconstruction of large areas, smuggling via secret tunnels, conversion of old quarry tunnels into storage cellars, mass reburial of the dead in underground ossuaries, and so forth. However, while the sewers beneath the two districts of the city are certainly completely unconnected, rumors claim that the older, deeper layers of the Undercity include secret passageways (perhaps sealed against groundwater seepage by the arcane tricks of dwarven masons) that run all the way under the river and connect the portions lying beneath the High and Low Cities.