I won't post a picture of cleavage here.
For pictures see 胸の谷間 - Wikipedia.
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First I had to google for a translation to my native language to understand what a cleavage is. Out of curiosity I checked what Google Translate gives. It gave me intermama fendo, which describes right away the matter! A further googling revealed that this time Google seems to have got it right.
Intermama fendo shows one of the strengths of Esperanto, an expression that clearly describes a matter instead of resorting to too specific words.