The Boiler Room is a solo map. It is playable as a net map, but it's mighty crowded already. It's a practice area, with plenty of bad guys to keep you company and its own special (meaning harder) physics model. It's like the Star Trek Holodeck or the X-Men's Danger Room. It's great for a quick splash of Marathon without having to fire up a scenario, pick a level, or open a save. I like to use it to kill a few minutes here and there while downloading. It's handy for helping to keep the old skills up to snuff. I hope you've brought game, because this place WILL kick your ass.
Levels in map "Boiler Room":
Levels in map "Boiler Room (no physics)":