Natural vanillin is extracted from the seed pods of Vanilla planifolia, a vining orchid native to Mexico, but now grown in tropical areas around the globe.  Madagascar is presently the largest producer of natural vanillin.
As harvested, the green seed pods contain vanillin in the form of its β--glucoside; the green pods do not have the flavor or odor of vanilla.
After being harvested, their flavor is developed by a months-long curing process, the details of which vary among vanilla-producing regions, but in broad terms it proceeds as follows:
First, the seed pods are blanched in hot water, to arrest the processes of the living plant tissues.  Then, for 1–2 weeks, the pods are alternately sunned and sweated: during the day they are laid out in the sun, and each night wrapped in cloth and packed in airtight boxes to sweat.  During this process, the pods become dark brown, and enzymes in the pod release vanillin as the free molecule.  Finally, the pods are dried and further aged for several months, during which time their flavors further develop.  Several methods have been described for curing vanilla in days rather than months, although they have not been widely developed in the natural vanilla industry, with its focus on producing a premium product by established methods, rather than on innovations that might alter the product's flavor profile.