Just as the future of a light carrier is an

<Why is Russia aiming nuclear use in the mountain valleys of the United States, where there are not many people?>

Just as the future of a light carrier is an unmanned aerial vehicle (UAV) light carrier, not a manned aircraft carrier, the future of a submarine is an underwater drone, UUV.

However, compared to UAV, UUV has a critical problem, and how to communicate and control it. Even if you do real autonomous navigation and attack, shouldn't you deliver an order to at least attack the enemy in a certain area? How do you deliver an order to UUV in salty water where radio waves are not working?

When using Extremely Low Frequency, radio waves can penetrate as far as several tens of meters below the water's depth. In fact, all nuclear-powered submarines transmit them to their allies in case of emergency through such ELF communication.  

Instead, ultra-low frequencies are more difficult to make, so the transmitting antenna needs to be hundreds to thousands of kilometers long. Is this possible? It is possible! We can make and use the earth itself as a kind of dipole antenna. The problem is that the length of the land is very important to build such an ELF antenna. That's why neither the UK nor France have ELF. So far, only the US, Russia, China and India have ELF facilities.  

Figure 1 below shows the structure of the Jim Creek Naval Radio Station in Washington, U.S.A. by cutting out large trees throughout the valley in a real mountain range and hanging cables 2-3 kilometers long on one side in zigzag to build an antenna. Those pylons in Figure 2 are 44 meters tall.

However, the actual transmission element is not a two to three kilometers long cable that is hung horizontally, but a cable that goes vertically to the ground from there. Long cables that are hung horizontally are elements to increase capacitance.

The scary part is that it's just a VLF (very low frequency) antenna, and the ELF antenna is much larger. Built in 1953, the Jim Creek Naval Radio Station Valley is still an active facility that can be used today, and according to Russian media reports in 2019, it is one of Russia's main targets of attack in case of emergency.