Until November 2000, patent applications could be prolonged artificially by the applicant. They remained secret until they "pop up", being granted and given a normal term, even when technology has advanced far beyond the contents of the patent. This strategy is profitable when the invention is widely used by the time of the issuance.
Those patent applications are secret and USPTO can't publish a list of the pending applications. Is there at least a number known of how many patents filed before November 2000 are still "submarine"? Is a FOIA request possible?