Screen shot 2011-08-16 at 2.08.41 PM

The default border is formed by block. Acid, acid rain, and virus cannot penetrate block. Ants trying to eat seeds, however, are able to penetrate block, as visible here.

Block is a fixed solid which is normally spawned and is impervious to acid, acid rain, and virus.

Block is the element used for the border.

Surprisingly, even Block isn't impervious to ants. If they are eating seed, they will be able to penetrate it.