Base of glass is dark How do I get rid of that nasty black thing at the bottom of the glass?

Shouldn't it be lighter than that? It disappears when I move the plane further down, but I need it close to the glass.

Any ideas?

http://imgur.com/a/u3USN -- far away plane.


http://imgur.com/a/dE0hO -- close plane.
This is likely happening because the light is not able to bounce back trough the glass. 

If you increase the number of transmission light bounces in your render you should solve the problem, also make sure Caustics are turned on.

They probably dont need to be as high as shown below so experiment a bit.