וְהַ֤ר סִינַי֙ עָשַׁ֣ן כֻּלּ֔וֹ מִ֠פְּנֵי אֲשֶׁ֨ר יָרַ֥ד עָלָ֛יו יְהוָ֖ה בָּאֵ֑שׁ וַיַּ֤עַל עֲשָׁנוֹ֙ כְּעֶ֣שֶׁן הַכִּבְשָׁ֔ן וַיֶּחֱרַ֥ד כָּל־הָהָ֖ר מְאֹֽד׃
Now mount Sinai was altogether on smoke, because the LORD descended upon it in fire; and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.
Read kabbalistic (mystical) interpretation of Exodus 19:18: and classical mystical sources.