A vast network of underground waterways and other waste disposal conduits, presumably connecting the Phobos facilities. It seems likely these corridors provided the demons with quick accessways into each facility, allowing them to overrun the entire moonbase as fast as they did. Deimos may have housed a similar network.
Other points of interest include the two teleporters guarded by Barons of Hell and a legion of Demons, bringing the total amount of Barons on Phobos to four. This demonic garrison is similar to the one seen at Phobos Anomaly, which implies the location was of some importance to the Spider Mastermind's plans. It seems these teleporters are entirely of Hellish origin and were constructed at the time of the invasion; it may very well be this site was going to be a staging area for the invasion of yet another location in the Solar system.