。
Thunder roared in the sky.
Literal
Thunder [subject-が] sky [in-に] thundered.
轟く ('to roar / boom / resound') is a literary verb, mostly applied to thunder, cannons, applause, or fame. It's a borrowing of mimetic origin (compare ごろごろ for thunder). 雷鳴 ('thunder-sound') is a Sino-Japanese compound contrasting with the native 雷 (which can mean either lightning or thunder).