Wonder where the cork in the wine bottle comes from?

I was fascinated by the cork oak trees and the cork harvest in Portugal. Although I had a vague idea that cork came from plants, until I visited there I had no idea how cork was harvested. Portugal produces approximately half of the world's cork.
The trees that it comes from are lovely.

The bark can only be removed every 9 years by law to protect the species. It doesn't harm the tree, and the cork bark regrows, so it is a sustainable crop. The year of harvest is marked on the trunk. This bark was stacked near the train tracks at Santa Clara a Velha, Alentejo, ready to be shipped.