rupture hardened structures is greatly decreased by the distance from the impact point of the nuclear weapon. Therefore, a nearly direct hit is generally necessary, as only diminishing returns are gained by increasing bomb power. The only purpose for gigantic nuclear weapons like the SS-18 25 megaton variant is to take out extremely hardened targets, like command-and-control facilities, such as NORAD, located at the Cheyenne Mountain Complex; Federal Emergency Management Agency (FEMA), located at Mount Weather; or Site R, located at Raven Rock. (The amount of energy needed to rupture missile silos is orders of magnitude greater than the amount necessary to destroy cities, which made the SS-18 25 megaton variant effective for the destruction of large urban centers, as well.) This could be a useful weapon for a decapitation strike, but that is a very risky move, and both the U.S. and Russia have extensive countermeasures against such methods. Anti-first-strike countermeasures According to the theories of nuclear deterrence and mutual assured destruction, full countervalue retaliation would be the likely fate for any state that unleashed a first strike. To maintain credible deterrence, nuclear-weapons states have taken measures to give their enemies reason to believe that a first strike would lead to unacceptable results. The main strategy relies on creating doubt among enemy strategists regarding nuclear capacity, weapons characteristics, facility and infrastructure vulnerability, early warning systems, intelligence penetration, strategic plans, and political will. In terms of military capabilities, the aim is to create the impression of the maximum possible force and survivability, which leads the enemy to make increased estimates of the probability of a disabling counterstrike, and in terms of strategy and politics, the aim is to cause the enemy to believe that such a second strike would be forthcoming in the event of a nuclear attack. Second strike One of the main reasons to deter a first strike is the possibility of the victim of the first-strike launching a retaliatory second strike on the attacker. Increasing SSBN deployment Nuclear-powered ballistic missile submarines (SSBNs) carrying submarine-launched ballistic missiles (SLBMs), commonly known as "boomers" in the US and "bombers" in the UK, are widely considered the most survivable component of the nuclear triad. The depths of the ocean are extremely large, and nuclear submarines are highly mobile, are very quiet, have virtually unlimited range, and can generate their own oxygen and potable water. In essence, their undersea endurance is limited