Superportal

A superportal is far more powerful than a normal portal. It is spawned after catastrophic quantum events such as the meltdown of the Citadel core, and can be used to ferry entire armies from one dimension to another. This was how the Combine overwhelmed Earth's forces during the Seven Hour War, and they planned to do so once more after the City 17 uprising began, but were foiled by Arne Magnusson, Eli Vance, Isaac Kleiner, and other scientists at the Resistance's White Forest base.

An additional fact is that superportals spawn Portal Storms in the surrounding area, making it incredibly dangerous to anyone near the superportal.