The Phantom Mask/Gate Map

First Floor:

Name: Mission Lobby
Depth: 0
Level Link: This mission's lobby is standard.

Second Floor:

Name: Creeping Cavern
Depth: 1
Level Link: Creeping Cavern is unique to this mission.

Third Floor:

Name: Emergency Outflow
Depth: 2
Level Link: Emergency Outflow is unique to this mission.

Fourth Floor:

Name: The Lab
Depth: 0
Level Link: While not actually listed in the mission's gate map, The Lab is always loaded upon completion and it has a special layout related to this mission.