This is the 4852nd most frequent Japanese word.
澄ん
"澄ん" means "clear" or "pure," typically referring to something like water or air.
In this sentence, '澄んだ' is used as an adjective derived from the verb '澄む', meaning 'to clear' or 'to become transparent'. Here it describes '空' (sky) as being clear.
澄んだ空を見上げた。
I looked up at the clear sky.
Here, '澄んでいる' is the present continuous form of '澄む', suggesting a state of being pure or clear, metaphorically applied to '心' (heart).
彼の心は澄んでいるように見えた。
His heart seemed to be pure.
In this sentence, '澄み切っていた' uses the verb '澄む' combined with '切る' to emphasize the total clarity of the water. It metaphorically or literally describes '湖の水' (lake's water) as being completely clear.