I find it difficult to accept the thought that they simply vanished. The little information we have about ancient civilizations seems to indicate that they had reached a level of technological advancement that could have allowed them to resolve whatever came from global geophysical disaster. They may have moved and from where they are now pose a threat to the current powers that be. This might explain why so many man-made disruptions occur all over the world. While humanity believes it is aimed at population reduction, these induced events could be caused intentionally to attempt to keep those in power that are spawn of those who remained on the earth's surface during the great deluge that eradicated almost all life on this planet. It could be an ancient war in which mankind is caught. The true cause of which man is kept in the dark about by those who chose not to live in the light.
Of course all this is mere speculation and it may be incorrect to a certain level. Just as there are colossal flaws, deliberate omissions and blatant lies in official historic records and in current mainstream media news reports. But where as the latter sources are instructed to intentionally create a false image of earth's past, this intuitive impulse is founded on an exact opposite aim. One time we will know or perhaps we already do without being aware of it.