In Russian, подполье (podpolye), "underground" (our translation), is the space beneath the floor of a house, formed from под, "under", and пол, "floor". It gives its title to Part One, and it is there that the offended mouse hides, in a disgusting, stinking underground. The narrator toasts the underground at the end of Part One and promptly sends it to the devil; in Part Two, he says he has carried it in his soul since he was twenty-four.
Where It Appears
Quotes
There, in his disgusting, stinking underground, our offended, beaten, and mocked mouse plunges at once into a cold, venomous, and above all eternal rage.
Part One: The Underground, the underground man
After all, gentlemen: it is better to do nothing! Better conscious inertia! So, long live the underground!