It would cause a very big tsunami, then the waters would go ere they came.
There is also a story about a volcanic eruption that caused all but 2000 humans to find out if there was an afterlife. That supposedly happened 75000 years ago.

No. Read Genesis 9. There is no sufficient evidence to justify the comet theory, and a real comet would destroy the earth entirely, altering its orbit around the sun, or rotation, both of which account for a perfect environment suitable to human life. This modern fable is about as accurate as the Babylonian fable.

Wow, thats one mighty comet, if it where the size of Mount Everest.

Actually, if a giant comet was involved, an impact was not necessary for a global-scale disaster. A "dangerously close" passage is enough to cause some "unusual" seismical activity and big tidal waves. Interestingly, Bruce Masse's assumptions reminded me the mysterious "Phaeton Myth" and Franz Xaver Kugler's intriguing essay on it back in 1927, titled "The Sibylline Starwar and Phaethon In the Light of Natural History." For more information on Kugler's work, see this document.