(Due to a lack of Green portals with Twilight involved in opening them I've decided to use the following picture from a comic made by FireBrandKun. The picture is a cropped piece of the comic and full credit of it's creation goes to him.)

Twilight has been studying portals she calls 'Rifts in the Veil' otherwise called Veil portals. After the appearance of humans in Equestria, she set out to learn and find out much about these portals.

Now she has the opportunity to test an experiment on one. Against her better judgement, she travels to the Crystal Empire alone to test her experiment on a Veil portal that has been sighted in a cave near the Crystal City.

But could this experiment backfire and send her through the very thing that she is studying?