able abysmal abyssal bottomless cavernous colossal deep as hell enigmatic enormous fathomless gaping immense incalculable incognizable massive mighty monstrous mysterious plumbless plunging prodigious puzzling sealed soundless strange tremendous unapparent unapprehended unascertained unbeknown uncharted unclassified undisclosed undiscoverable undiscovered undivulged unexplained unexplored unexposed unfamiliar unfathomable unheard unheard-of unidentified uninvestigated unknowable unknown unperceived unplumbed unrevealed unsounded unsuspected untouched vast virgin without bottom yawning