US Navy submarines can receive messages while submerged, but can't respond without raising antennas and exposing themselves. That's why Lockheed Martin is developing a system of buoys that will be able to relay surface signals to submerged submarines:
The idea was to excite the upper atmosphere with high-frequency radio waves, and it would then emit the ELF bands required for one-way communicate with submerged submarines.
The new system is the first two-way communication method for submarines at depth. The actual depth is classified, but according to Reints the cables attached to the tethered buoys are “measured in miles” and would allow them to be launched from “a significant depth.” The submarine could continue normal operations at its normal speed while communicating.
Link via DVICE | Image: Lockheed Martin