Ground wave is a method of sending messages over a distance while keeping the fact that you're signaling somewhat hidden. It's essentially radio, but it uses the Earth as a medium instead of the atmosphere. An antenna is sunk into the ground, and radio frequencies designed to reflect off the surface (and thus remain underground) are used. A similar antenna stuck in the earth will be able to receive these signals. The more built-up an area, the less likely this will work; foundations, pipes, conduits, etc. all act as gigantic interference sources in much the same way a building blocks your cellular call.