Sub-Zero Containment is a room within the Main Deck of the B.S.L research station. The small room is a cold containment cell, holding nothing but the frozen carcass of Ridley. If Samus visits the room before obtaining the Varia Suit from the Mega Core-X, she will take damage from the cold environment due to her Fusion Suit's susceptibility to it.
The room's only function appears to be to hold Ridley's body. Whether he was dead or alive before the X attacked the B.S.L research station is unknown, though considering it is a below freezing temperature room, Ridley may have been frozen here.
When the Main Deck's main elevator ceases function while Samus is on it, Samus must break through walls to escape the elevator shaft. She passes through Sub-Zero Containment on her way out. When she does, a Core-X emerges from Ridley's body and leaves the room.