N2
中を
なかを
Through, amid (making your way through a place or hard conditions)
人混みの中を歩くのは大変です。
It is hard to walk within a crowd of people.
Explanation
「Aの中を」 is followed by a movement verb and means 'making your way through A'. A can be a place (「人混みの中を歩く」 walk through the crowd), but it is usually an unpleasant condition: heavy rain, a snowstorm, a busy time. It suggests pushing on despite the difficulty, and is also used to thank someone for coming when they are busy: 「お忙しい中をお越しいただき、誠にありがとうございます」 (Thank you very much for coming despite your busy schedule), 「雨の中をわざわざ届けてくれた」 (He took the trouble to bring it through the rain). 「箱の中を覗く」「鍋の中を洗う」 are just 中 'inside' + the object marker を, not this pattern.
Mixed up with
「〜中を」: must be followed by a movement verb such as 「行く」「歩く」「走る」「お越しになる」; it feels like pushing through.
「〜中で」: only a place or range, no movement through it: 「雨の中で立っている」 (standing in the rain).