A Room.
You are in a room.
The floor is made of concrete.
The walls are made of concrete.
The ceiling is made of concrete.
There are no doors, windows, or other openings. Somehow, the air in the room is being refreshed constantly.
You have no idea how you, or anyone else, got in this room. You don’t see much of a way out. There’s nothing here except the clothes on your back – even the light is just a dim ambient illumination, with no visible source.