Deep basing is a strategy for basing MX
Peacekeeper ICBMs so as to maximize their
survivability. It is brought to you by
The Reagan Administration, the color brown, and the letter $$$.
Since the MX missile silo was designed to be extremely hard (see densepack for more info), the main issue that arose in deciding how to base it was how to ensure that it would be able to respond even if the Soviet Union 'overkilled' it by allocating lots of warheads to its location. One such method was 'deep basing'. The notion was that the missile would be buried in a silo deep underground, with no immediate (vulnerable) access to the surface. In the event of a nuclear strike, systems emplaced in the silo would be used to 'burrow' upward after the attack until the missile could reach open air, at which time it would be launched. This would allow the earth above it to shield the missile from even multiple direct hits with nuclear arms, at least in theory.
This plan foundered for many reasons; one primary one was that no one seemed to be able to describe a workable means of burrowing out that wouldn't involve explosives. In addition, the problem of how to emplace the silo and missile that far down would have been extremely costly to solve.
See also: Densepack, Rail-mobile, Reverse Inclination Basing.