Originally announced in the September/October 1990 issue of Nintendo Power magazine were both NES and SNES ports of Will Wright's highly acclaimed SimCity, a city simulator and one of the first games of its kind. Both of the ports were slated for a Spring 1991 release and while the SNES game was released as planned (albeit being pushed back to an August 1991 release to coincide with the US release of the SNES), the NES port was cancelled shortly beforehand and never ended up seeing the light of day.
In the November/December 1990 issue of Nintendo Power, two screenshots from the NES port of the game were released, along with a short rundown of the said port. It received a few additional mentions in the months to follow (including, notably, a couple of additional magazine write-ups and a short preview video) and was eventually forgotten by most (assuming that the game had simply been cancelled), until 2005, when, in the "Collecting" section of an issue of Nintendo Power, it was revealed that a prototype cartridge of the NES port had recently been rediscovered in the desk of Nintendo Power editor Scott Pelland. The article revealed that the prototype cartridge was housed in a hollowed out golden The Adventure of Link casing (including pictures of the actual cartridge) and revealed that the game was cancelled due to "bad timing" (assumingly referring to the aforementioned 1991 US release of the graphically superior SNES and its corresponding SimCity port).
Since the port's 2005 rediscovery, it has been kept tightly under wraps and a ROM has never been leaked online. It is currently unclear as to exactly who is now in possession of the prototype cartridge, though unless said owner decides to share with the rest of the world, this rare and highly sought-after port will likely never be played by the general public, or so we thought.
Later in 2018, it was finally dumped online.