어느새 (eo-neu-sae) – before one knows it
This word is useful to describe occurrences that happen without your knowledge or to describe something very sudden.
You can also use this to express how something happened faster than expected, which is quite similar to 생각보다.
아까 비가 많이 왔는데 지금은 어느새 그쳤네.
It was raining really heavily just now but now it stopped before we knew it.
우리는 매 번 모으고 술을 하자면 어느새 새벽이 되네.
Every time we gather and drink together, it would be daybreak before we knew it.