What does "知らざった" mean?

[seanblue]

We had a somewhat technical discussion about ざる over in the Aria book club last week if you’re interested.

Aria the Masterpiece: Chapter 44 Discussion

Though I can’t say I understood its usage in this chapter since the dictionary definition seemed weird in this context. Also, Aika using ざる on page 113. 灯里の知られざる一面が見られるかも. “Maybe we’ll be able to see an unknown side of Akari” Is that right? What’s the difference between using ざる and ぬ in this context? I found two answers ([1]; [2]) but other than saying ざる and ぬ are both 連体形 or ず, they weren’t very helpful (took me forever to figure out what 連体形 was too).