One of my favorite recent video games is Subnautica, in which you have to survive almost entirely under a vast alien ocean. You have the advantage of advanced technology, but even then you are under constant threat of running out of oxygen, or having your habitat implode because it was not sufficiently reinforced. You are mostly working in brief increments and shallow depths.

That is similar to reality. Underwater is an extremely challenging environment, and human researchers do most of their work in brief increments and shallow depths. It says something that we have a continuous human presence in space, but not underwater. Some of the challenges are similar. You need a protected environment that is largely self-sufficient, at least for long periods of time. You need to provide heat and oxygen, and deal with changes in pressure. Of course, in space the problem is the lack of pressure outside the space station, while under water the challenge is increased pressure. This is actually far more of a challenge underwater, as the deeper you go the pressure difference from inside to out can be far greater than in space. Underwater you also have to deal with the corrosive effects of salt water.

A company, Deep, plans to have long term continuous human habitation under the ocean for the first time, with their Sentinel habitat. This will allow up to six people to live at 200 meters under the ocean for up to 28 days at a time. They plan on deploying the crew in November 2026. The Sentinel will be 400 cubic meters in size, with modules 6.2 meters in diameter – that is about the same diameter as a 777 fuselage. Imagine living in a space the size of a large jet for 28 days. By all accounts the space will be nice, but that is still cramped an isolating.

At this point the most salient question is – why bother? We can send robots underwater to do research and bring back specimens (a similar argument to why put humans in space). While true, the Sentinel will be at 200 meters depths, in the pelagic zone. This is too deep for any significant diving from the surface, it is not in contact with the shore, and yet is home to 90% of sea life. While robotic undersea research has been great, they do not completely substitute for a human researcher. The ability to stay under the ocean and observe the environment and conduct research for long periods of time is valuable.

We can also argue that the experience of living in the Sentinel is another way to prepare for human trips to deep space. Whether under the ocean or in space, living in a small tube for extended periods of time is psychologically stressful. There is a Mars Mission simulator, and four people have been there since June 25th on their one-year mission. I do wonder, however, if the fact that this is a simulation makes it less psychologically impactful. They know they can, if absolutely necessary, just step out the door.

In Sentinel, however, you are truly isolated, in a hostile environment, and can’t just leave. When problems arise, they are real, and could threaten your survival. Likely, we will gain useful experience from the undersea “aquans” (as they are being called) that will translate to space travel. Perhaps NASA will one day send their astronauts down to Sentinel for training.

The Sentinel will also have a “moon pool” (just like in Subnautica) which is essentially an open-bottomed chamber so you can just descend into the water. The water is kept out by the air pressure in the chamber, which means the air pressure has to be the same as the water pressure at that depth (or else the water would enter and just compress the air). So the aquans would have to pass through an airlock to the high pressure moon pool, where they can freely access the ocean. They would then have to go through decompression to reenter the rest of the habitat.

If the Sentinel project is successful, what would that mean. At the very least, it will be a great platform for oceanic research, allowing researchers to stay for prolonged periods of time in a critical zone of the ocean. It will also dovetail nicely with deep space travel research. Will it lead to anything else, however?

I seriously doubt that there will be large communities living underwater in the foreseeable future. There are simply too many downsides. Undersea bases will be limited to research for now. But I can see there being a future for undersea bases being used for tourism. However, this will likely remain a niche industry, much like the deep sea tourism industry is now. Taking a sub to a station where you can stay for several days, having larger windows and better lighting, is more appealing that taking a small sub with a small window for mere hours. It’s also probably safer, as the subs themselves can be optimized for safety rather than viewing, and the station is a much larger construct that does not have to go through cycles of compression and decompression. And, you can go diving through the moon pool. That’s likely to be a vastly superior (and expensive) tourism experience than a one-off sub ride (although you can’t visit the Titanic that way).

At the very least I hope the Sentinel proves to be a boon to oceanic research. The ocean is a vital resource and key to our environment, and the more we understand what is happening to the ocean, especially in a world changing through human action, the better.