I cleaned up a mess related to one going off about 15 or so years ago in San Jose. It was overfilled with cooling oil. There are lines denoting how much to fill one with the oil based on temperature and load, and a not to exceed point. Someone topped it off. The failure point was all of the bolts, spaced about 1-2 inches apart, all the way around a circular access hatch about 2 feet in diameter. Every single one gave way it appeared, simultaneously. If there had been someone standing around it they would have died a hideous death being soaked in deep fryer temperature oil...
Personally, for years I have been concerned about the vulnerability of those whopper big KVA transformers in substations and their design dependence on oil for cooling and dielectric duties.
Not going to say any more...
And THATS why I don’t go in transformer vaults. Most I will do is look in from the door while shielding my body behind the door jam.