We know more about the surface of Mars than the bottom of the ocean, or so I hear a couple of times a year. I don’t know if that’s true, but it sounds believable because we see more news about space. Space is a wonderment; we can see it from anywhere. The bottom of the ocean is covert, covered, we can’t see it, even though it is a lot closer. The heavens seem close, because we can see a lot of stars from almost anywhere. Most of us only see the edge of the ocean. Or the top. Where the water is very clear one might see the bottom 600 feet down. Most places you are lucky to see 6 feet down and the average depth is over 2 miles. There is twice as much of Earth’s surface under water than above.
Some are concerned about running out of resources. Air should be our first concern, because a human can only live about 3 minutes without oxygen. Fortunately, the air supply, although not as clean as it once was, is truly vast. Not only does air cover the entire planet surface, but to a depth of about 60 miles. We are not likely to run out although we have been pretty successful at poisoning it. Politicians are starting to listen to science about the danger of continuing to add pollutants to our air. If we explore other worlds, the Moon, Mars, or the bottom of the ocean, we have to bring or create our own air. We might be able to extract oxygen from space rocks using prodigious abouts of expensive energy. On Earth, oxygen is entrained in the ocean and as fish demonstrate can be extracted relatively easily.
Water comes next, because one can only live about three days without it. The Moon and Mars have none that we can prove. We could extract the necessary chemicals from the rocks and recombine them into water, again at tremendous energy cost. The bottom of the ocean is covered with water. True, it’s salt water, but the technology to separate it is old, simple and well understood. Transporting surface water to undersea locations does not involve space-age technology.
In terms of food, humans have exploited the seas for food so long that we can’t say when it truly began. Was there ever a time without fishing? Food in space, probably, will have to be transported from Earth until hundreds of technical and agricultural impossibilities are overcome. Don’t hold your breath.
The other resources we depend on come out of the ground, minerals that are essentially the same below the sea as above. We obtain many from below the sea already, particularly oil, gas and diamonds. Many of the mineral deposits on the ocean floor are just that, deposits lying there on the bottom where they can be collected rather than mined. There are environmental issues, but remember the ocean area is twice the land area and there is little life to disrupt at extreme depth. There is a temperature gradient from which energy can be extracted easily and underwater hot spots, free energy.
Science fiction makes interstellar travel seem doable by wishing away a few stark realities. Unlike travelling to space,we can commute to the deepest part of the ocean. Surface to bottom can be measured in hours rather than weeks, and that is just if we are going to the moon. Surface transit across the ocean, takes hours by air, days by ship; a blink of the eye compared to the months, or years we’d need for interplanetary travel. Can humans live underwater? Some already do, submarines have been virtually everywhere in the top thousand feet for more for 100 years. Going bigger or deeper is just a matter of cost. We go undersea in other ways, tunnels have been bored from dry land to nearly 1000 feet below the sea surface. So far this has been just for transportation but other applications are foreseeable. The least practical way to live beneath the sea involves pressurization, caissons. Unfortunately, humans do not tolerate being pressurized for long, but longer than they can tolerate the zero pressure of space. There is a lot of opportunity, a lot to learn, and we have barely started, even though Jules Verne imagined it 150 years ago.