Undercity

The Undercity is what Caelestrans call the caverns beneath the city. Most of what has been charted are the ruins of the ancient Asurian city which modern Caelester was built on top of, but even before the earthquake, several passages into deeper caverns had been discovered. These natural-seeming caverns are considerably more dangerous than the ruins, and had never been fully explored. Since the earthquake, many new passages opened up, leading to entirely new chambers. Indeed, they are so numerous that some scholars suggest these represent something far more than just a series of natural caverns beneath the city: the fact that the caverns seem like they should overlap and intertwine, and yet fail to do so implies the possibility of some planar magic at work, or perhaps an illusion of staggering complexity and power.  

Sections

Asurian Ruins

While no part of the undercity is entirely safe, the Asurian ruins seem to be the most hospitable to visitors; or perhaps it would be better to say they are less hospitable to those who dwell in the deep places of the world. Over the long history of Caelester, most of these ruins have been explored. However, the earthquake did open up several chambers and passageways that none had guessed even existed.   Most of the city's sewer system passes through these ruins, flowing through sealed brick channels designed to ensure an efficient flow of effluent to its final destination. This has the happy side effect of allowing both maintenance crews and explorers to avoid the unpleasant sensations that accompany sewer traversal in other cities.  

The Caverns

Beyond and beneath the ruins are a number of caverns that would seem natural, if not for a number of magical challenges that plague those who attempt to explore them. Some seem to shift over time, with maps made on one expedition failing to match the caverns encountered on a subsequent expedition...or in a handful of cases, shifting during visits, squeezing shut paths behind the explorers while opening new passages beyond. Several contain segments of magical darkness that renders sight impossible without conjuring forth powerful light magic. And some contain Decay Fields, pockets of necrotic energy that cause any who stumbles into them without appropriate protection to rapidly age and decay. One of these fields exists very close to the ruins, and is used by the Guild of Nightsoil Collectors who manage the city's sewers as the end-point of the sewers. They have discovered that the material produced by this rapid decay forms a thick, blackish brown oil, known as Naphtha.  

Even Deeper

Some explorers who venture deep into the caverns claim to have found a distinct section of tunnel. Descriptions vary dramatically - some report chambers seemingly constructed entirely of human bones, while others claim to have found a hellish chamber populated by gigantic mutated insects, shadows that cut armour and flesh alike, and glistening spikes that smash together like grinding teeth. Most believe these individuals to have simply gone mad in the darkness before eventually finding their way out, but if such a layer exists, then it is certain these few were lucky to have survived whatever is down there.