Hell was one of the map themes used during Minecraft's Indev stage, and was eventually removed in the Infdev stage. The concept of a hell world re-surfaced when the development of Alpha 1.2.0 began, with the introduction of the Nether.
Description[]
Hell resembled the Overworld of that time, only much more apocalyptic. Unlike the Overworld, "Hell" generated lava in place of water. The sky would also appear red, instead of blue. This map theme made gameplay harder than if the player spawned in the Overworld, as lighting levels were lower than in the Overworld - this ultimately resulted in more mobs spawning. Farming also took much longer, as there were very few trees and a low supply of natural materials.
Most Minecraft players never knew this feature. It looked similar to the Nether Waste (as it was the Nether’s early development stage), but the Nether wasn't added at the time. Another key difference is that because of the lava and lack of water in "Hell", if the player got burned, there would be no place to cool off, and the player would eventually burn to their death there.
Due to the many mobs that spawned here, mining trees in Hell was difficult due to interferences from hostile mobs. There was no way to build a hut or a house without mob interference.
Gallery[]
An aerial view of Hell, from Mojang AB