[Sax]

Of course one has to ask, where did the pool water go? It was irradiated yes? Probably into the ocean?

Damaged pool, water leaked out?

Heat of rods more intense than expected, water boiled away?

In desparation, they rerouted/siphoned the water to another reactor when it was perceived to be a bigger problem?

[buzzer]

“Damaged pool, water leaked out?
Heat of rods more intense than expected, water boiled away?”

Probably both. The Pool at reactor 4 might have been damaged from the explosion in Reactor 3. And the pool of reactor 4 contained hot fuel due to the maintenance of reactor 4.