For some reason, I can't move or remove an 'invisible' cube which is on my map. I searched on Google to find a solution but somehow I still can't remove it.

The cube looks like this: http://screencloud.net/v/uNyz

In Brush Wireframe: http://screencloud.net/v/3C0c

In Wireframe: screencloud.net/v/oGBj

As you can see, I want to delete the brown cube. Selecting it and pressing the DEL button won't do anything.

So, how do you delete the brown cube?

EDIT: Seriously, I wrote this post a few minutes ago and I found the solution. However, I still don't know how to delete the brown cube.