On the morning of the third day there were peals of thunder and lightning, and a heavy cloud over the mountain, and a very loud blast of the shofar, so that all the people in the camp trembled. But Moses led the people out of the camp to meet God, and they stationed themselves at the foot of the mountain. Now Mount Sinai was completely enveloped in smoke, because the LORD had come down upon it in fire. The smoke rose from it as though from a kiln, and the whole mountain trembled violently....
Exodus 19:16–19