Balls: I went to my macbook air a few moments ago and discovered what you see in the photo. At some point, I don’t know when, the battery expanded sufficiently to destroy the case. It’s a known weakness of lithium ion batteries at their end of life. The computer was not being charged at the time: overcharging is apparently a common cause of battery swelling.
I have to admit I’m surprised I heard nothing. I saw the machine a couple of hours earlier when it appeared fine. I’d not left the building, yet I heard nothing. I’d have hoped something as visually nasty as this would at least have caused a nice loud crack. Come on, battery, if you’re going to destroy a computer, you could at least have put on a show.
To be fair to Apple, I bought the computer back in 2011, so it’s given me 13 years of solid use. I could replace the battery, apparently, but I’m rather confident that a 13 year old macbook air will not be the easiest machine to fix. I’ll just replace it.