In the heat of battle, the United States military coordinates air and ground movements using a global communications network of satellite links, encrypted wireless networks and old-fashioned radios. But what happens if over-the-air transmissions are jammed or hacked by enemy operatives? Take it underwater, of course.

DARPA, the research and development arm of the Department of Defense, is in the testing phase of a deep ocean, fiber-optic network that would restore tactical communications if conventional satellites and radio frequencies are knocked offline.

Code-named TUNA (Tactical Undersea Network Architectures), the backup communications system would consist of a network of radio buoys strung together by hair-thin fiber-optic cables floating hundreds of feet below the ocean surface.