The Underworld is rightly named such, although it is also called the Afterworlds, the Lower Umbra, or Dark Umbra. It is home to the Restless Dead, the wraiths who still cling to the world because of something left undone. Very few have the desire to travel here. This is the Umbra of memory: emotion invested in things which no longer exist. Even wraiths themselves can be said to be creatures of memory and passion – although whether it is the memory of those still alive or of the Wraiths themselves which keeps them alive is unknown. In the Middle Kingdom, this Umbra is known as the Yin World.
There is a gravity of sorts in the Underworld, but it has nothing to do with mass. Instead, all things are drawn inevitably to Oblivion, the end of all things.