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Thanks everyone, I'll qualify my response in that Beso's theory is clearly official, but the extra qualification of the eventual end result is also important as we simply can't know the timescale here and if the oceans can retain heat hundreds of years or longer it shouldn't automatically be considered as a short term possibility. Also the heat of the deep ocean from volcanic vents etc is a vast temperature and must contribute as a constant source of internal heat eventually working its way out as well.