"The Russian government daily Rossiyskaya Gazeta reported that to achieve ‘extensive radioactive contamination' the weapon ‘could envisage using the so-called cobalt bomb, a nuclear weapon designed to produce enhanced amounts of radioactive fallout compared to a regular atomic warhead,'" Schneider said.
Retired Air Force Gen. Robert Kehler, former commander of the U.S. Strategic Command, has said development of the underwater nuclear strike vehicle is one element of a "troubling" Russian strategic nuclear buildup.
Rep. Mike Rogers (R., Ala.), chairman of the House subcommittee on strategic forces, has said that the Russians assert the nuclear drone submarine will be used to target coastal areas and inflict "unacceptable damage to a country's territory by creating areas of wide radioactive contamination that would be unsuitable for military, economic, or other activity for long periods of time."
Russia calls the system "Ocean Multipurpose System 'Status-6," and it is allegedly capable of traveling underwater to distances of to 6,200 miles. It can submerge to depths of 3,280 feet and travel at speeds of up to 56 knots.
The Pentagon has confirmed that a new Russian nuclear delivery drone is real. The undersea drone, which carries an enormous nuclear warhead to destroy coastal cities and military bases, was tested late last month.
Reports from Russia indicate the bomb could be armed with a "salted bomb", or one that "salts the Earth" with the dangerous isotope Cobalt-60. Such a bomb could spread such high levels of radioactivity it would prevent anyone from using the attack zone for approximately 100 years. Depending on location and prevailing weather conditions, such an explosion would also carry vast amounts of radiation inland.
Update: Nextbigfuture looked at megatsunamis trigger from landslide displacment and the possible effect of megatsunamis on the US East Coast.
What if XKCD examines the issue of waves from nuclear bombs.
The seminal work in the field of nuclear ocean waves is Water Waves Generated By Underwater Explosions, a 400-page report produced for the Department of Defense by Bernard Le Mehaute and Shen Wang. The report, published in 1996, exhaustively examines and summarizes all available research about the ocean waves created by nuclear explosions.
The report outlines how when a nuclear weapon goes off underwater, it produces a cavity of hot gasses, which then collapses. If the explosion happens near the surface, it can create some pretty big waves—under some circumstances, they can be hundreds of feet high near ground zero.
Devastating tsunami within 100 miles and strong tsunami out to 500 miles
SOURCES- Free Beacon, Popular Mechanics, Dep Defense - Water Waves Generated By Underwater Explosions, XKCD