During your In Sound Mind journey, you’ll come across several puddles of toxic sludge the player can’t wade through. Within minutes of starting a new file, you’ll see a gas mask through a window, but the door beside it is locked. As it turns out, you don’t need a key to get into that room.
Unfortunately, the gas mask won’t be within reach for around two to three hours. For starters, you’ll need to complete Virginia’s tape, which is the first major playable chapter exploring the psyche of what will be multiple patients from Desmond’s past. This tape and its associated reward are unmissable.
Upon finishing her tape, a shard of glass becomes a permanent part of your inventory. Use the facility’s elevator to return to the basement floor. Cut the police tape immediately to the left from exiting the elevator with the newly acquired glass shard. From there, you can step onto a barrel at the top of the steps to reach a vent. Break it with the glass shard and proceed to the next room through the vent.
This next room presents a miniature platform challenge in which you must keep yourself from touching the toxic sludge-infused floor. Get to the end and you’ll find the gas mask resting at the familiar table. From here on out, you can equip the mask using the same trigger you use to aim your handgun or peer into the glass shard.
You can’t exit the room via the way you entered as the vent is too high to jump to. The room’s door doesn’t budge either, leaving you to smash a vent by the door on ground level with the glass. Congratulations! Your shiny new piece of equipment will allow you to explore more optional bits within the game.