The Corrupted Sector
This mod adds a new dimension called The Corrupted Sector. It is very similar to the Overworld, except it shows signs of being corrupted on disk: chunk edges, missing chunks and 'barren' chunks that don't have top level blocks like grass or sand on them.
The dimension is intended to be used as a mining world of sorts: to be wrecked and drained of its resources without having to disfigure the Overworld!
To reach The Corrupted Sector, you'll first have to craft a Glitch Rift:
Simply place one of these down and right click it, and you'll be warped to the corrupt world.
If you get stuck in the dimension, try to find a missing chunk! Jumping into the void in this dimension will take you right back to the Overworld.
The 'corruptness' of the dimension is configurable, as well as the presence of clouds, the day/night cycle and how much the world seed differs from that of the main world.