Exodus 19:18

19:18 Now Mount Sinai was completely covered with smoke because the Lord had descended on it in fire, and its smoke went up like the smoke of a great furnace, and the whole mountain shook violently.

Exodus 20:18

20:18 All the people were seeing the thundering and the lightning, and heard the sound of the horn, and saw the mountain smoking – and when the people saw it they trembled with fear and kept their distance.