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So, in the mid 25th century, humanity has become a space faring race, with about 10,000 planets under control of Earth. There are many different ecosystems on different planets, from normal ones, like Earth, to the craziest ones you can imagine.

On the planet Oceanus, a civil war has erupted between two powerful clans, the Skillsi and the Kar’Qom. The Kar’Qom are backed up by a rival of the humans, the Qualian Star Empire, which plans to annex the sovereign planet.

The humans start helping the Skillsi, but there is a problem. The planet Oceanus is mostly aquatic, with only a few islands larger than 100 miles. The human soldiers will need to get genetic alterations to be effective soldiers, but in the UTF there is a law that states “No major changes can be made to a human being through genetic modifications” which means they can’t add extra organs or make the person miss the appearance of a human.

So, considering these laws, how could you make soldiers that could operate underwater?

  • The majority of the cities are between 160 and 300 feet underwater, where light can mostly reach.

About the Oceanians

The Oceanians are a crustacean like species, who evolved from an ancestor living in tidal pools and coral reefs. They have 21st century level technology, like nukes and submarines, which function like cars. They live in mostly isolated city states, in tight-knit communities

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  • $\begingroup$ Have you considered to just give them scuba gear? $\endgroup$ – Philipp Jun 2 '18 at 21:31
  • $\begingroup$ @Philipp: They will need to be underwater for extended periods of time $\endgroup$ – DT Cooper Jun 2 '18 at 21:32
  • $\begingroup$ Why do you even need soldiers. Given the sheer amount of resources (9999 planets) humans have you could just throw amphibious mechanical mercenaries at your rival until they tire out. $\endgroup$ – Suhrid Mulay Jun 3 '18 at 5:09
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You will need to provide oxygen. If you do not want them to carry it you will need to produce it from the water. There are 2 options I can think of.

1: Extract oxygen from the water like a gill. This works for fish. You would need to extract a lot to have enough to breathe. There exists a device which purports to do this now. https://www.deeperblue.com/triton-underwater-breathing-gill-scam/ An artificial gill would mean a large surface area device on or behind the soldier which processed water and extracted oxygen. The device would need to recirculate inert gas to dilute inspired O2 content to a safe level so your sea soldiers would still need to carry gas.

2: Split water into hydrogen and oxygen. This is a breathable mix. It is also explosive, but you are unlikely to encounter sparks in your lung. It would require considerable energy inputs but a small volume of liquid water becomes a very large volume of gaseous hydrogen and oxygen. This would be 33% O2 which is higher than surface normal but that is fine. The other 66% is hydrogen. A benefit to this is that this mix (minus some % oxygen) should be safe to breathe at depth/pressure; like helium, hydrogen should not cause narcosis.

Bonus - soldiers breathing freshly split H2 and O2 would exhale the same gases + a percentage of CO2. If you had a flame valve on your exhaust (or perhaps a catalytic reconverted) you could convert exhausted H2 and O2 gas back into water, and so produce no bubbles.

  1. For mobility and warmth you would need an exosuit with built in swim motors.
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Using genetic-engineering, you should be able to crank up the overall cell respiration, so that only anaerobic respiration is enough to provide the needed energy and implement a system that gets rid of all the lactic acid, and boom, you created a human that doesn't have to breathe.

Alternatively, you could use some kind of advanced rebreather.

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You could emulate the dive biology of the sperm whale as a template for your submarine infantry's genetics. A quick read of the sperm whale's sensory make-up could also offer some solutions to the logistical restrictions of individual soldiers having to communicate with one another under water without having the ability to exhale.

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Annihilate the enemy by sheer mechanical force. You could 'circumvent' the UTF law by completely foregoing the idea of modifying your humans and simply start mass producing & dropping aquatic vehicles to pulverize all opposition. This can even be an exercice in muscle flexing, showing off humanity's strength/industrial power against their rivals.

Given that humanity in your setting already controls about 10.000 worlds, it's safe to say they should have the technical knowledge to create machinery to suit the environmental needs of most planets. Humans probably have encountered dozens of ocean worlds, ice worlds and maybe even gaseous ones where they were forced to manufacture "sea"-going vessels.

possible inventions:

  • giant underwater motherships with modified 'research' drones

  • multi-purpose flying ships that can operate in aquatic environments as well as in the air

  • stealth orb-like mini-death stars piloted by humans that shoot super-heated jets of water

Of course, this turns the humans into something of a brute-force-ish ally, but cooperation between them and your aquatic race could make an efficient partnership where humans provide the firepower and the Skillsi specialise in flushing out the hideouts.

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