Suppose in a post-nuclear apocalypse world, there are still less than a million survivors throughout the world. Radiation levels in the air are lethal for many animals, sea water is contaminated and there is a persistent stench of burnt humans everywhere. Most survivors are hiding beneath the ground, having stayed clear from the blast as well as the searing heat.
Could radio work underground? Suppose there are still functional signal towers transmitting messages. If not, how would survivors communicate at a distance when anyone will become unconscious on the surface in 30 mins?