Diesel locomotive backup generators
I consulted at a company once that had a big datacenter with a loading dock off the back door. Off that dock there was a short hallway with very thick steel doors on either end that led to huge concrete room that housed massive banks of forklift batteries powering their UPS, and two diesel locomotive engines serving as backup generators. The datacenter only needed one to run the DC + HVAC, but I was told it used to require both back in the mainframe/mini days.
One weekend they were doing some regularly scheduled maintenance/testing (which I was not involved with) on the UPS/generator setup. One of the locomotive engines threw a piston or some other important part, which impacted the other locomotive engine and caused it spray debris that impacted the ceiling, wall, electrical and HVAC equipment along with a number of the giant batteries. Fortunately no one was in the room at the time it happened or they almost certainly would not have survived.
It took them most of the weekend to clean things up and replace or bypass damaged electrical components to the point where they could re-power the datacenter, then start bringing servers back up. This last part was my only involvement, I didn't do 'on call' but they needed all hands to help to get things back up by Monday morning and I was happy to lend a hand and make some easy money.
Their only backup at that point was the (remaining) batteries which would only cover a very minor interruption. It took several weeks but they were able to find someone to rebuild the first locomotive engine that started the whole mess and get it operational again. The second was declared a total loss. Fortunately they did not have any power outages during the time they were without backup generation.
I wish I'd gotten a copy of the pictures from the guys who were involved, they were both scary and impressive.