system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do Naval submarines stay in contact with the mainland?

I know that regular frequency waves have an extremely hard time penetrating water, and I know that the lower the frequency is, the less feasible it is to submerge the entire transmitter. So how do submarines stay in contact with the surface? Say if a nuclear strike was ordered?
gpt
Your intuition that low-frequency radio waves are the only means of transmitting through water of any significant depth is correct, as is your intuition that the antennae required to transmit such radio waves are too large to mount on submarines. But you're missing something pretty significant: the antennae required to *receive* these radio waves is no bigger than any other receiver.

This means that submarines can still *receive* VLF radio transmissions (circumstances permitting) even if they can't transmit them. Of course, the fact that we're talking about VLF means that the bandwidth is so low that only text communication is practical. There's not enough bandwidth to broadcast real-time audio signals, let alone video. We're talking 300bps. A high-end consumer-grade broadband connection can be upwards of 300**M**bps, literally a *million* times faster. 

That said, VLF radio waves only penetrate to depths of perhaps sixty-odd feet, so a submarine at its normal patrol depth would not be able to receive transmissions without ascending. Sometimes this is what happens, but for subs that stay at depth for months at a time, this is clearly not an option. One solution is radio buoys, tethered to the sub and floated to transmission depth while the sub itself remains far below. But the long and short of it is that a sub at patrol depth will probably be out of contact with the mainland most of the time. 

For what it's worth, most *two*-way communications with subs is via the same satellite communications network used by other naval vessels. This does require subs to surface, or at least ascend to a depth shallow enough to permit the exposure of an aerial/mast, but it's pretty much the only effective means for submarines to *send* messages, not just receive them. 

There are also rumors of acoustic communications systems maintained by the US and Russia. These would involve acoustic sensors/transmitters being set up at specific points on the ocean floor that nearby submarines could use to send and receive messages that way. The existence of acoustic communications networks has not been definitively confirmed, but even if they did exist, they would be of very limited utility. These would presumably be encrypted so as to avoid interception, but that's not the only problem. The mere *fact* of a transmission would tend to indicate a nearby sub, and even that much of a hint is something most sub captains try to avoid at all cost.