[neil 47]

Paul

Gravity pulls the moisture into the carpet and gets trapped between the underlay and carpet backing .

It then gives off a smell due to bacteria build up and struggles to evaporate due to being trapped and no matter how good your vacs are its to late .

Fans and a dehumidifier help but I've never bothered just left it 6 to 7 days and it's always gone

[John Kelly]

Its mildews not bacteria that are usually responsible for the musty odour. Thats why spraying a bacteriacide deoudoriser is not recommended as you are just adding more moisture.
Surfaces dry by the moisture evaporating into the air above. If the air above is already at its maximum moisture holding capacity then no drying will take place.
Also you get a layer of wet air about a centimetre above the surface called the interlayer. Above this is where the evaporation takes place. Air movers work by disturbing that interlayer and causing more rapid evaporation.

[Simon Gerrard]

Someone knows what he's talking about