Scientists have come to the conclusion that - somehow - some time - for some unknown reason - undersea volcanoes must have erupted en masse and caused oceanic extinction. Well, except for the timeline, the whole theory gets a big fat DUH! Christians, and Jews before them, have known for a few thousand years that "the same day were all the fountains of the great deep broken up..." "And all flesh died that moved upon the earth, both of fowl, and of cattle, and of beast, and of every creeping thing that creepeth upon the earth, and every man: All in whose nostrils was the breath of life, of all that was in the dry land, died."
It's in the Book, man! Just read the Book, save yourself a lot of trouble guessing around. (Genesis chapter 7)