der Keller
basement / cellar
The lowest part of a building, usually located below ground level, often used for storage.
Wir lagern unsere Winterreifen im Keller.
We store our winter tires in the basement.

basement
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basement / cellar
The lowest part of a building, usually located below ground level, often used for storage.
Wir lagern unsere Winterreifen im Keller.
We store our winter tires in the basement.

Pay attention to the case after the preposition 'in': 'im Keller' (in the basement, dative) describes a place, while 'in den Keller' (into the basement, accusative) expresses movement there.
Die Waschmaschine steht im Keller.
The washing machine is in the basement.
Things are often stored in the basement. Typical verbs are 'lagern' (to store), 'aufbewahren' (to keep/store), or 'abstellen' (to put/set down). Common nouns are 'Weinkeller' (wine cellar), 'Heizungskeller' (boiler room), or 'Waschkeller' (laundry room).
Viele Leute lagern alte Möbel im Keller.
Many people store old furniture in the basement.
The idiom 'im Keller sein' (to be at rock bottom) is colloquial and means that something is at a very low level. The subject is usually something abstract like 'die Stimmung' (the mood), 'die Moral' (morale), or 'die Motivation' (motivation).
Nach der Absage war seine Laune den ganzen Tag im Keller.
After the cancellation, his mood was at rock bottom all day.
The colloquial phrase 'in den Keller gehen' (to plummet) describes a rapid drop. It is often used for 'Preise' (prices), 'Aktienkurse' (stock prices), or 'Temperaturen' (temperatures).
Über Nacht sind die Temperaturen in den Keller gegangen.
Overnight, the temperatures plummeted.
to fall
'fallen' and 'der Keller' can both express that something is becoming lower or is very low. 'fallen' is a normal verb and describes downward movement, for example with prices, temperatures, or numbers. 'in den Keller gehen' (to plummet) is more colloquial and often emphasizes that something drops sharply or unpleasantly. 'im Keller sein' (to be at rock bottom) describes more the low state afterward. For neutral information, 'fallen' fits better; for everyday language and a stronger effect, 'Keller' sounds more natural.

Nach der schlechten Nachricht war die Stimmung im ganzen Team im Keller.
After the bad news, the mood of the whole team was at rock bottom.
Nach dem Sommer fallen die Temperaturen oft schnell.
After summer, temperatures often fall quickly.