2 . Some talk of building settlements on the moon or mars to help make sure humanity survives long into the future. Others have their sights set closer to home: on future cities under the ocean.

“Technologically speaking, it is absolutely possible to colonize the bottom of the sea,” says Fabien Cousteau, a well-known ocean explorer who once spent 31 days living in what is now the world’s only functioning undersea habitat, Aquarius. It’s about the size of a school bus and located 62 feet below the ocean surface off the Florida Keys.

Just as astronauts test what it would be like to live in space, aquanauts try out undersea living with an eye on the future. Escaping from disasters such as climate change, wars, or pandemics is one reason to live under the sea. Others include studying, exploring, or appreciating the ocean. It’s costly and expensive to constantly keep up an ocean habitat which must pump in or make its own air, electricity, and fresh water. Inhabitants have to deal with high pressure. And infections can develop rapidly due to high humidity.

Despite the challenges, several new ocean habitats and cities are under construction. A series of space station-like undersea living quarters for aquanauts called Proteus are scheduled to be completed by 2025 off Curacao, an island north of Venezuela in South America. An undersea colony called Ven Base Alpha will be built near a hydrothermal vent, which would supply it with energy. Meanwhile, an undersea city called Ocean Spiral in Japan would link its surface to the deep sea, using the difference in pressure to generate energy and produce fresh water. This sounds amazing, but it would be extremely expensive and remains just a design.

Not everyone agrees that building homes in the ocean is a good idea. This type of development could place even more strain on ecosystems that are already struggling with pollution and climate change. It’s important to make sure that any construction is done carefully, says Susanne Menden-Deuer, an oceanographer at the University of Rhode Island. Wild undersea habitats should be treated as “the precious, irreplaceable resource that they are,” she says.