A multi-story house is held at the top of a square shaft 5 meters by 5 meters by 100 meters down into the Earth. Explosive bolts hold the house at the top of the shaft. The house is square, about 4.9 meters by 4.9 meters and 12 meters tall. When detectors on the roof detect photons from a nearby asteroid hit or H bomb explosion: The bolts blow, and the entire house begins to descend into the shaft at 16 meters per second per second. Since this is faster than gravity, very short burn rockets are needed on the roof to help push the house downward for 1/4 second. The house decends 1/2 meter and the loose items in the house, including the people are levitated to about 1/5 meter off the floor. Next, the house descends at 9.5 meters per second = about 99% of the acceleration due to gravity, for 2 seconds. Lesser rockets will be needed to maintain this acceleration due to air resistance. The house descends 19 meters for a total descent of 19.5 meters. This means the house is clear of the top of the shaft, so the blast doors can finish closing about 1/10 th second later at t = 2.35 seconds. The loose stuff and people have returned to the floor. We next allow the people 1/10 th second to recover their balance t = 2.35 seconds at normal gravity, but the house is falling, so we need to hold the decent rate constant at about 23 meters per second to produce the sensation of not falling. The average acceleration for the first 2.25 seconds was about 12 meters per second so the speed of falling is 27 meters per second. From t= 2.25 to 2.35 the house falls 2.7 meters to a depth of 22.2 meters. We now need to decelerate the house to zero with respect to the bottom of the shaft. If we decelerate at 9 meters per second, deceleration time is 3 seconds and the people experience almost twice normal gravity during the 3 seconds. The house decends another 40.5 meters during the last 3 seconds to t = 5.35 seconds and 62.9 meters down the shaft. We should likely think about 100 meters as a safety factor, to allow gradual transitions and the possibility that the blast will accelerate the house down the shaft if it arrives before the blast doors fully close, or the blast doors fail due to the blast.
Only 2.25 seconds warning of the blast are needed and the shelter is the house the family lives in.
The house is small and the shaft may cost more than a million dollars to construct. An extra room is needed at the bottom of the shaft for survival supplies and equipment. A sloping tunnel is needed to the surface when it is safe to return to the surface.