From Halopedia, the Halo wiki
Crisium City is a city located in Mare Crisium in the province of Naniwa on Luna. The city is notable for being the birthplace of the Rookie and Michael Crespo. The central office and production plant of AMG Transport Dynamics is based out of Crisium City.
Mare Crisium is a real location on the moon located on the near side facing Earth. It is a large "sea" much like the famous landing site of the Apollo 11 mission, very flat and over 500 kilometers wide it would be a good location for a lunar colony. Its location can be seen in this image.
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