Hammers fans are used to hearing announcements belted out over the club's sound system at home games exclaiming 'Mr Moon is in the building' and 'Mr Moon has left the building'.
The announcements are always greeted with cheers when Mr Moon arrives and boos when he leaves, from the Upton Park faithful.
It's the club's biggest in-joke, except very few know the punchline.
The announcements have become so iconic they even made it into one of EA Sports' FIFA football computer games.
While some 'in the know' supporters may already be wise as to exactly who, or rather what, Mr Moon really is, for thousands of West Ham fans it has been a mystery stretching back decades.
Well the question has now been put to bed by none other than Jack Sullivan, the teenage son of the East Londoners' co-owner David.
And it appears the mysterious Mr Moon isn't a Mr at all, it is in fact a signal to club staff when one of the fire alarms has been set off.
That's not to say West Ham fans should suddenly start rushing for the exits fearing for their lives when they hear it from now on.
Sullivan has reassured supporters it is an alarm which always goes off.
Responding to a fan asking who Mr Moon was on Twitter, Sullivan said: "If a fire alarm goes off, one in one of the kitchens that goes off always, therefore the firefighters come (Mr Moon)."
If a fire alarm goes off, one in one of the kitchens that goes off always Therefore the firefighters come (mr moon) https://t.co/c5j1aiOxrI— Jack Sullivan (@jsullivanwhu) November 11, 2015
So there you have it, word from (near) the top on Mr Moon. The next question is, will he be moving to the Olympic Stadium as well next summer?