I was in my church's gym the other day with the 1000 watt MH/Merc high bays and a weird thing happened. It was during a storm and the power went out. I went in the electrical room(friends with the coaches) and turned off the lights. Soon the power came back on. When it did, I went to fire up the lights and everything was normal(the fixtures were warming up as usual) when all of a sudden I heard a "whoosh" noise and when I looked to see what was making the sound one of the highbays was shooting flames!     Within a few seconds, the flames stopped and then the light went out. I then notified the coach and he reported it to the custodians to fix. ( maybe I can get the bulb if they change it!) What happened? My guess is the ballast took a dump. Any guesses? 

More then likely something happened that shorted the ballast during the storm, I've seen an HPS ballast catch on fire from a shorted ballast before! is the lamp still intact?

Yes the lamp is still there. I hope to get it when the fixture is fixed! 1970's 1000 watt MH lamp.

how many highbays are in there?

20, 5 on each bank, with 4 bank(rows). Always fun to start up, you have to pull these big breaker switches. Pretty colors too!

It's very highly likely, like 99.9% likely a 1970s MH lamp is long gone, since older MH lamps didn't last as long as the today's ones. So the MH lamp you saw in there is probably a newer lamp. Also in the 70s highbays were still mostly MV. They may have also been converted. Even some 80s highbay and indoor fixtures were MV! My high school has a newer wing built in 1987-1988 and the new wing has some recessed square lights with clear MV lamps!