Cloned Cube is a Portal mod. It takes place after GLaDOS' fall in Portal.
You assume the role of Chell's secret clone.
You have to escape or else you may just be tested to death.
There is one obstacle.
CUBiS is trying to kill you.
The Underground section of Cloned Cube has been updated tremendously. Here are some of my select screens that I like.
The rest: Minus.com
Video: Moddb.com