Plumes of dark smoke are spewing from underneath the car. Source of smoke?? on 1995 Mitsubishi Diamante

Car runs great. There are no apparent problems with the exhaust system, or at least not the pipes or muffler. Can't really see the rear header that well. No unusual exhaust noise. Car does not use oil. Check engine light not on. Most common during hot weather.
About 127K miles.