I have only played the Single Player part of the game and have not tried playing any of the user created maps. There were also rumours back in the day about this and that that one could find in the game, some were true, others were hoaxes. You could try a search on google seing if you can find any info about alien motherships and the moon god.
The only alien mothership I rember having seen in a game is in Fallout 3.
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