Welcome back to our series where we ground our vision for a closed-loop city in the lessons from real-world experiments. Having secured a breathable atmosphere, we now address the solvent of life itself: water. In any sealed environment, recycling water isn't just a good idea—it's the only path to long-term survival.

Welcome back. In our last article, we introduced our two guiding stars for grounding our closed-loop city in reality: the ISS, our pragmatic engineer, and Biosphere 2, our ambitious gardener. Today, we put them to the test on the most critical and immediate challenge of any sealed habitat: creating and maintaining a stable, breathable atmosphere.

Welcome back to the Project Clean Up blog. Over the past several weeks, we have outlined the ambitious vision for a completely closed-loop city—a self-sustaining, regenerative model for humanity's future on Earth and beyond. We've explored its theoretical systems for water, waste, food, and energy.

Now, it is time to ground that vision in reality.

The dream of creating a self-sufficient human habitat is not new. We are fortunate to stand on the shoulders of giants—pioneering projects that have tested the limits of science and human endurance. For our next series of articles, we will be guided by two of the most significant experiments ever conducted in this field: the International Space Station (ISS) and Biosphere 2. They are our real-world blueprints, teaching us through their incredible successes and their invaluable failures.