You are looking for a bison tube. BYOP.
This series of caches is placed to bring attention to the " Morse Code". While it is not used any more as a primary form of long distance communication it is alive and well in the amateur radio (Ham) community. The code is also used in emergencies. When all other forms of communications are gone Morse Code can still be used to communicate. The use of sound, light, or any method of making dots and dashes can become life saving in an emergency. An example of this would be an "SOS", sent as di di di dah dah dah di di dit -three dots, three dashes and three dots.
.