Outer Expanse

Outer Expanse is a region added in the Downpour DLC. It's separated from the rest of the world by a locked Karma Gate in the westernmost place of Subterranean. The gates were placed under lockdown by to keep the creatures of the place from ransacking his and 's facility grounds. It can be only explored by at first once they meet Five Pebbles. Once their ending is completed, the and the  gain access to the region as well.

Overview
The Outer Expanse is a lush and lively place, dotted with ruins of the Ancients. This place is also home to a number of unique creatures, the best known being the Slugcats.

Facility Roots (Western Intake)
The campaign starts in Outskirts right after it exits the Facility Roots.

Journey's End
The tree-home of the Slugcats seen in and  opening cutscenes can be found here.

Trivia

 * This region is the original homeland of the Survivor and the Monk, and quite likely the natural habitat of the species.