Read this and answer the question

An exception is the United States, where patents filed prior to 8 June 1995 expire 17 years after the publication date of the patent, but application extensions make it possible for a patent to issue much later than normally expected (see submarine patents). The various MP3-related patents expire on dates ranging from 2007 to 2017 in the U.S. Patents filed for anything disclosed in ISO CD 11172 a year or more after its publication are questionable. If only the known MP3 patents filed by December 1992 are considered, then MP3 decoding has been patent-free in the US since 22 September 2015 when U.S. Patent 5,812,672 expired which had a PCT filing in October 1992. If the longest-running patent mentioned in the aforementioned references is taken as a measure, then the MP3 technology will be patent-free in the United States on 30 December 2017 when U.S. Patent 5,703,999, held by the Fraunhofer-Gesellschaft and administered by Technicolor, expires.

A patent issue much later than normally expected can be referred to as what?
submarine patents