No, it isn't a long lost episode of Star Trek, but Ned Kahn's “tourbillion” installation art, a collaboration with architect Uwe Bruckner, for the Duales Systems Pavilion at Expo 2000 in Hannover, Germany.
This “dancing airborne funnel of whirling fog,” we read here, was 7 stories tall; powered by large turbines; fed with fog from an ultrasonic humidifier; and observed from a ramp wrapped around the cylindrical atrium.
What a Marvelous sight it must have been.
But as it is, we can only now see it writhing and screaming in this unfortunately compressed video. Fortunately, videos of Kahn's other works are available here. Similarly lo-res, but better than none at all.
There is also The London Tornadium.
Tornado Alleys of Mars