Darker Places: Descent

Hey all! Yet another addition to my Darker Places series. I don't know if I've stated it here, but I plan to paint some of these. Watercolor, gouache, maybe acrylic. I'm still thinking of practicalities. Anyhow, my new addition is yet another city; the infernal twin-city of Descent.
The name Descent is almost a joke amongst the population. The city in fact has two true names, one for each wall of the massive gulf that makes up the city proper, but everyone simply refers to the city as Descent. The vast architecture of both halves of Descent honeycomb the stone walls and dig themselves deep into the earth. Both halves of the city function independently of one another, and even trade with one another as if they were separate lands. Each half of the city has its own customs, arts and beliefs. And schemes. Though there have been conflicts in the past due to the polarized nature of the twin-city, they have usually been brief and subtle. Descent's true strength is something that both sides of the city can share, and it is also what keeps the city-states separate: the gulf.
For all intents and purposes, the gulf is bottomless. Descent has carved itself deep into the walls that oversee the gulf but only to a point. After a certain depth, construction of the ever-expanding city peters out, and few will willingly stay close to the gulf for long. The gulf is a place of deep mystery: a source of incredible power and even greater danger. The scientists of Descent have learned to harness the powers that seep from the depths of the gulf and bend it to their wills. The city and its industries (largely based around stone and gem-work and some metallurgy) are powered by energies drawn from the gulf. However, the gulf is not treated as a bottomless put, but rather as a divinity. Despite the temptation, the citizenry of Descent do not callously throw objects into the bottomless abyss, not so much as trash. To do so is considered a taboo and even accidental dumping must promptly be followed by an absolving prayer, begging the powers of the gulf for their forgiveness. People of Descent are largely separate because few people dare to cross the gulf. Once every twelve days, bridges are cast over the gulf allowing trade and transit, but any other movement across the gulf outside these times is generally forbidden. The only alternative is to leave the pits of Descent and trek across the barren wasteland that lies at the top of the gulf; an inhospitable plateau of searing winds and sand-blasted rock, and perhaps worse horrors still. The trip is hard and takes several days.
What lies inside the gulf that its citizenry would fear it so, yet still dare to tap into its wellspring of power?