The population of humans not living on land will exceed the population living on land. The non-surface population will live in floating cities, underground, on the bottom of the ocean, in orbiting space stations around Earth, the Moon, and Mars, as well as on the surfaces of the Moon and Mars. Potentially, there will also be populations living underground on Mars and the Moon if it turns out that living below the surface is more practical. There will likely be a few floating space stations between Earth and the Moon, and between Earth and Mars, and perhaps a few beyond Mars.

Analysis: As humanity approaches a population of 10.5 billion by 2100, we will begin to seriously explore how to fully maximize Earth and local space. The urgency will be driven by longer life and technological advancements that make our current AI aspirations look primitive. In preparation for the challenges of building permanent floating space stations beyond Mars, humanity will construct massive floating cities and underground cities, followed soon after by aquatic cities on the ocean floor. Simultaneously, we will perfect space station technology, including orbiting, surface, and underground habitats focusing on the Moon, Mars, and its two moons.

By the time humanity reaches biological immortality by about 2200, life in these exotic areas will become “normal.” People will die mainly from accidents or a refusal of medical treatment. They will live large portions of their first century in ocean-floating cities, underwater stations on the seafloor, and space stations. Starting in about four generations, some people will be born, raised, and die without ever having seen land on Earth. Their reality will be shaped by their environment, whether it is one of the ocean cities, an aquatic station on the sea floor, or a space station within view of Earth.

By 2350, within another ten generations or so, our bond to terrestrial Earth will be replaced with a bond to where they were born. As always, some people will migrate from one place to another for various reasons.

Viewpoint: The time from now to then will feel as distant as the Middle Ages feel to us today. Future generations will look back at us in the same way we look back at figures like Michelangelo, Copernicus, and William Shakespeare.

Vacations: Options for yearly vacations will include traditional natural destinations like forests, beaches, and islands, as well as entertainment-based destinations like Las Vegas, Disneyland, and Paris. If you live in one of the orbiting space stations or on the Moon, you will have access to your own Disneyland and Las Vegas attractions. Someone growing up in one of the aquatic stations on the ocean floor might venture yearly to the surface.

Beyond yearly vacations, once-in-a-lifetime adventures lasting nearly a year will include experiences like a Martian adventure, where you travel to the Moon, visit the large space stations between Earth and Mars, and explore the space stations orbiting Mars and its moons, Phobos and Deimos. You could then either visit the surface of Mars and/or its moons or venture out to the large space stations established beyond Mars. Naturally, your choice for a once-in-a-lifetime trip will depend on where you grew up. If you grew up on Mars or one of the distant space stations, your once-in-a-lifetime vacation might be a trip to Earth.

Timeline: 
2025-2035: Initial Martian and Lunar Bases. Space Tourism Begins.
2035-2050: Expansion of Orbital Stations. Underwater Habitats.
2050-2100: Space Station Network. Underground Cities. Increased Non-Surface Population.
2100-2150: Advanced Lunar and Martian Colonies. Permanent Underwater Cities.
2150-2200: Orbital Habitats. Interplanetary Trade and Travel. Cultural Shift.
2200-2250: Massive Space Stations. On-water, underwater, and underground cities.
2250-2300: Population Growth. Advanced Technologies.
2300-2350: Self-Sustaining Habitats. Interplanetary Colonization.
2450: Non-Surface Population Tipping Point.