The nuclear device in Carl's inventory, deliberately withheld from the abyss and saved for something specific.
Elle McGib suggested Carl find a way to use the nuke sitting in his inventory to clear out The Abyss, reasoning it would do the job. Carl declined: he was saving it for something very specific. The floor was ended with The Nightmare Express's satchel charges instead, leaving Carl's Doomsday Scenario entirely intact and unspent for a purpose he has not yet disclosed.
A nuclear device carried in Carl's inventory.
The crew's reserved doomsday option, held back even at the floor's climax.