in such a situation; amidst that; therein
Used to refer back to a previously described situation, often introducing a contrast or continuation. Functions as a conjunctive phrase.
See also: その中
In the midst of that, he was quietly reading a book by himself.
会議は混乱していた。そんな中、彼女が突然立ち上がった。
The meeting was in chaos. In the middle of that, she suddenly stood up.
A fixed expression composed of そんな (such, like that) and 中 (middle, midst). The exact historical derivation is uncertain, but it functions as a set phrase in modern Japanese.