THIS POST HAS BEEN BLANKED
THIS POST HAS BEEN BLANKED
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
It means you have a leak. Your map has a hole in the world brushes exposing entities to the black void. If you don't know where it is, go to map>load pointfile and block the hole that the red line shows you.
Ok so I did what you told me to do (Thanks for the leak tip, btw) but the line is aimless.
...and this is the line.
Not to be snarky, but I bet someone is going to say this map is every hammer crime ever.
Urge to stop mapping rising.
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
You seem to have stuff outside your map;
In the source engine you must have everything inside; so you must have walls and ceiling in order to get this to actually compile 
Well, for hammer it is. Your map is meant to be inside a box. If you have a large area, you need a large box. Simple! But you didn't do that. You're entire map is exposed. I'd recommend TopHATTwaffle's V2 tutorial videos on YouTube, because, you don't seem to understand how to get started.
No, my map is intended as an exterior building. There's the "GLaDOS Shutdown and Cake" Door on one side.
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
As I said, you need a large box. Texture the inside of the box with toolsblack. I think you should see the videos. The black void is not a pit BTW. Your map needs to be SOMEWHERE though.
That is the best answer i heard yet! 
_____
Just take a look into some of valve Maps, in the wheatley levels.
There you'll see, that the whole level, is sorrounded by a huge box. Outside nodraw, and inside toolsblack. Your map must be 'everytime' a "closed" level.
Here an Example:
I tried putting a wooden box around it to make it unexposed, but no dice.
PLEASE DON'T LOOK AT MY POSTS FROM 2013
THEY ARE UGLY AND HIDEOUS
I really like your mapping Style!
I will make you an own example, one moment.