Synonyms for Bombproof (same or very similar meaning)
WordNet sense 1 (a chamber (often underground) reinforced against bombing and provided with food and living facilities; used during air raids):
bomb shelter, air-raid shelterWordNet sense 2 (able to resist the explosive force of bombs and shells):
shellproofWordNet sense 3 (immune to attack; impregnable):
invulnerableFrom the ODE community, based on WordNetadd/edit