Fountains of the Deep
Hat tip to Donald Sensing
Apparently there’s some water in the mantle. About an Arctic ocean’s worth, they guess—or about 1.4% of the surface water. It sounds plausible. I’d wondered where subducted water went, and so did they.
Of course, if that were to reach the surface there’d be hell to pay. Luckily it could only rise slowly—if it came up in one big gusher/fountain it’d raise ocean temperatures by 20 degrees C (35 degrees F)—eventually. Of course that much steam in the air would fry every air-breather long before the oceans reached equilibrium.