Special door in the Vatican: Only Open Once/Century?

I read once about a special door, in the Varican City. It is only opened when the Pope proclaims a “jubilee” year-this is usually once every century.
What is the sgnificance of this door? Is it used to transport aything?

Cecil has done this. Here’s the column on it…

Wiki entry when you type in “Jubilee” and look at “theology”: http://en.wikipedia.org/wiki/Jubilee_(Christian)

Google result from “vatican holy doors jubilee”:
http://www.vatican.va/news_services/liturgy/documents/ns_lit_doc_20010106_chiusura_en.html

Holy doors.

There’s four of them.