basement

kellarikerros, pohjakerros

kivijalka A floor of a building below ground level.
(RQ:Vance Nobody)

Turning back, then, toward the basement staircase, she began to grope her way through blinding darkness, but had taken only a few uncertain steps when, of a sudden, she stopped short and for a little stood like a stricken thing, quite motionless save that she quaked to her very marrow in the grasp of a great and enervating fear.