How can a nuclear submarine cross an eldritch ocean safely?

Question

Submarines are now the sole method of communication and transport between allies on the different continents of Earth.

This is because eldritch monsters have begun appearing all over the globe, with the following consequences:

• All communication farther than 5 miles has been rendered impossible by the presence of these creatures. This includes wired connections.
• Aircraft instrumentation and radar has suffered a similar fate, and pilots are reduced to VFR at all times as well as using only dumbfire weapons.
• Aerial and surface threats are sufficiently dangerous that other methods are considered too costly or unfeasible. Sizeable surface fleets have attempted the crossing and been attacked so fiercely they were forced to retreat.

The creatures share many traits with ordinary sea animals, and can utilize all methods of detection that they have. Engagement with any of the lesser creatures is not certain death but should be avoided at all costs.

With submarines left as the only viable way of maintaining contact with remote allies, what strategies would best ensure their safety while crossing the oceans?

Answer 1

Meet your new friend, the thermocline.

This is a layer in the ocean where the temperature changes drastically. For submarines, this has long been an important aspect because it doesn't transmit sound well (the wikipedia article mentions this too and has more links for a deeper dive if you'd like). So for a submarine hiding from surface vessels, you'd want to be under the thermocline. If you're hiding from monsters under the thermocline, you'd want to be on top of it. But I feel it's reasonable to guess that if monsters are attacking surface ships, then they are probably above the thermocline, listening for the ships, and so as the sub captain I would treat it like trying to avoid destroyers. Stay under the thermocline. Maybe I don't know how they detect me but I'm guessing they do it by sonar (active or passive) and the thermocline should help hide me.

Fortunately you have a nuclear submarine so there's no need to surface. Nuclear submarines make their own oxygen so the only limit is how much food and supplies you brought. Packing enough for a trip across the Atlantic would be no trouble at all (nuclear subs typically pack enough for 90 days, from what I can scrounge around online, and I think even a careful slow boating across the Atlantic would take you less than 15 days, however you figure it. Cruise ships do it in about a week.)

It could be worth pondering some sonic attacks. Would pinging really loud scare off the monsters, or just attract more? What about a torpedo detonation? I wouldn't be in a hurry to test this, but there could be some things to try in an emergency, or, more ideally, well ahead of time, using drones.

We might also investigate if ELF (or other obscure submarine communication options) still works. These super low frequency options are not ideal for communicating (I have heard they are very slow) but maybe they'll get past monster interference? Worth a try. If nothing attacks the land based transmitter then we're good to go, at least for updating the submarine on where it needs to go.

(Edit for another thought: wait for winter, and make the trip under ice as much as possible. Reasoning that whatever the monsters are up to, hopefully they have a lot less reason to be hanging out underneath a frozen arctic, and the sub can try to make as much of its trip through there as possible, more or less, depending on where it's going. Probably not much help in getting to Crete but would be handy for a trip between New York and Norway.)

Answer 2

Movement and grouping strategy

Group your submarines together for two reasons :

• The more you spread them, the more likely one will get detected and destroyed. Indeed the area of one getting detected at any time will be directly proportional to the number of submarine "squads". Building submarines is expensive, losing one is like losing it permanently. During several trips this will be very damaging to your submarine economy.
• But most importantly, in case of contact you have more firepower, much more firepower. Firepower which can damage or even kill 0the enemy before they can hit you (long range torpedoes vs maws and tentacles, you know) This will make any encounter with "lesser" threats more easily manageable. With several submarines you can cover the vanguard's blind spots, too¹.

However, like you don't put all eggs in the same baskets, don't put all your submarines on the same trip. Pairs or triplets are largely enough. You don't want to lose all your submarines at once when they meet good ol' giant Ch'Thulhu, and you might want to transfer several messages at different locations anyway.

Then, without knowing the threat, I'd advise to move near the surface or the opposite, near seafloor. You'll remove one angle of attack, one which is a big weakspot for your submarines. This also cuts the detection sphere in half², reducing the risk of being found out. This and well, it will use the thermocline layer, as JamieB cleverly pointed out. If you can, choose trajectories closer to the equator, so that the thermocline is sharper due to the higher temperature difference.

Extend your detection range, know your enemy's behavior, keep your maps updated

Any tool to extend where you know your enemies are or will be is an invaluable asset. The more you know your surroundings, the easier it will be to avoid superfluous battles. Chart any previous engagment and detected threat, check for patterns and give the results to the next submarine captain so they act accordingly.

There is a caveat to the "detect as far as possible" approach however : Sonars work based on echolocation : This is an active detection system... Which can be detected by the very thing you want to avoid! If you know you are in a dangerous area. Stop emitting and use passive technology instead, slow down, and finally... hold your breath. I hope your heart can withstand the fear of the unknown :).

Create lures

Use autonomous submarines or special, non-explosive torpedoes as lures. They can be used to distract any incoming threat with something expandable. They should also be nice addition for recon' and vanguards for the same reason they can be lost with less cost.

Of course, they should use big speakers and play sea shanties to annoy them on low frequencies for greater coverage. The more attractive and tougher your lure is, the easier its usage. Alternatively, put a very strong explosive trap on the decoy, though note surviving Eldritch monsters will certainly remember it next time.

---

^{1 : I dismiss the responsibility for the following trauma in case the crew have to destroy another submarine to save their own skin.
2 : Effect lessened if you find there are now eldritch monsters digging in the ocean's sand or diving from above the surface. But that's an another issue :).}