Eltanin impact
I've seen energy calculations done in about 1968 that claimed such an event would create an enormous amount of heat-energy to "pastuerize most of the planet". Part of the hypothesis concernered studies of Verdevoort Ring in S. Africa & other large scale ancient impact sites, and concluded an object of this size would be sufficient, in a water strike, to crack the mantle, and release enormous amounts of magma, which would create a relatively long term additional vaporization. In turn, the vaporization would slow heat disipation and radiation to space. In effect, there would be an essentially dry area of ocean floor, with a wall of seawater surrounding it, and a rising column of superheated steam for a fair amount of time, until the seawater managed to cool the crater enough to reinundate it.
The results indicated a significant rise in atmospheric temperature, torrential rains & winds, followed later by 'atomic winter' type cooling. It was a scenario of a major global killing event regardless of impact point, assuming a strike in 1-3 Km depth ocean.
In addition, naturally, they mentioned the super-tsunamis this would also create.
The Eltanin impact did happen. And it had nothing to do with Atlantis, Mu, or the Sphinx. If you prefer not to receive GGG pings, let me know.