Unable to shut down the portal activity from the receiving end, you head over to Mars Nuke Facility to look for clues as to where the demons spawn in from.

As you enter the facility there is no doubt you've reached the right place, as the stench of nuke waste fills the air.

Level: Marks Nuke Facility
00:00 Major demon fight room
03:43 Level start
17:30 To Mars surface 

Note: Drat! Nothing like losing 70 minutes of fun gameplay 🤦‍♂️☹️ Short of replaying the level from an earlier save point to redo the level, here's a preview of the map along with the ending leading to the next level...

Basically, this level is good for getting the BFG 9000 so there's that I suppose 😛

Gameplay Playlist
https://www.youtube.com/playlist?list=PLH9X4m3s2fcdjQApDi1de9AGhjtwVTPA8

About the mod

Doom III is a Mod for the 1994 hit Doom II by id Software. This mod let's you get back into the fight against evil and it's gameplay is modernized while still playing very much the way Doom and Doom II did.

Fight your way through 2 Chapters of new levels, SinglePlayer or Coop. Play as Doom Guy, Doom Gal, or one of the other selectable characters. Customize your difficulty by chosing your Character, Skill and Save-Point amount. Alongside all the well known monsters, items, weapons and power-ups there are also lots of new ones, to help you fight back the demons.

Get the mod
https://www.moddb.com/mods/doom-3-doom-ii-mod
...
https://www.youtube.com/watch?v=a4Ikl_GkTVY