Why is it cloudy? Most Prosecco is made using use the Charmat method, where the wine is fermented dry, then undergoes a secondary fermentation in large steel tanks, kick-started by a carefully measured dose of sugar and yeast. The CO2 from this second fermentation is trapped as the bubbles we love. The wine is then bottled under pressure, clear, without the spent yeast cells that caused the bubbles.
"Col Fondo" wines are made using the Methode Ancestrale, where wine is bottled before it has finished fermenting, and sealed to prevent any further bubbles from escaping. When the fermentation is complete, the producer could choose to disgorge the wine (as with Champagne) and bottle it clear, or keep the spent yeast cells in the bottle. Col Fondo, meaning "with the bottom" is the Italian name for the style of keeping the yeast cells in the bottle, causing the familiar cloudiness.