DARPA’s AMEBA developing transmitters that could communicate through walls, undersea and underground environment

Modern day radio transmitters and receivers, the devices that uses electromagnetic wave signals to communicate through cellphones, radios and television, seem to be present everywhere.  The propagation of these electromagnetic waves has some limitation too,   Key among these is that radio frequency signals hit veritable and literal walls when they encounter materials like water, soil, and stone, which can block or otherwise ruin those radio signals. This is why scuba buddies rely on sign language and there are radio-dead zones inside tunnels and caves.