Sinkhole

Also known as the Helltown Sinkhole.

The Sinkhole is a gigantic sinkhole ( roughly 2 miles in diameter and 1 mile deep) that swallowed the city of Hilltown, and consequently a good chunk of the Grand Canal. Rumors abound as to what caused this to happen, but it did happen and many believe that the old city of Hilltown lies buried at the bottom of the Sinkhole.

Also noteworthy about the Sinkhole –
There are many Dungeon tunnels that can be accessed from the edges of the Sinkhole. The upper levels have been tamed by the denizens of Helltown as housing and places of work, both salvage and mining being popular pursuits.
It is widely rumored that there is an extra-dimensional element to the Sinkhole, this is born out by the finds of artifacts that are from wildly different places in space/time. Some say that you may be able to travel to other planes and dimensions through the dungeons of the Sinkhole.

Sinkhole

Adventures in a Decaying Empire Jagatai Jagatai