Iridescence caused by a single layer of oil spread out over water. I used to see this when I was a child on the school playground, I don’t know what had been spilt. I also remember seeing this sheen on the tarmac on roads when I was young. But now cars are less polluting the phenomenon has reduced. But obviously some cars still leak oil. Will this happen when all cars are electric? Doubtful, but who knows when that will be.