Describes fog that is heavy and reduces visibility significantly. Used literally and sometimes figuratively for a situation that is unclear or hard to see through.
深い霧で前がよく見えなかった。
I couldn't see well ahead because of the thick fog.
山道は深い霧に包まれていた。
The mountain path was enveloped in dense fog.
Compound of 深い (fukai, 'deep') and 霧 (kiri, 'fog'). The phrase is transparent and follows standard Japanese adjective-noun modification.