Originally posted by Dakota.
Already posted, and your welcime for showing you the spot
Please don't pretend like you discovered this first. For those who try to keep these locations private to prevent the whole world using them, I've been exploiting this location since the first day of Alpha. If you move back and forth between both spawn locations, or stand on the room with an Auto-Rifle and enhanced zoom, you can spawn an event where enemies spawn non-stop for a good 10-15 minutes, with multiple ultras.
Rinse and repeat for even greater results.