Where does the wealth go when asset prices go down?

Over the past couple of months, stock markets crashed, crypto assets crashed even harder, trillions of dollars gone. But what does «gone» actually mean here? Where did the money go? And if it just vanished, does that mean is was never there in the first place? It’s one of these things you might intuitively understand, but have a hard time explaining to someone else. Not Noah Smith. He makes it easy to understand (and even fun to read).