Concentrating on the developmental challenge of food security, production can take place locally in urban areas, i.e. exactly where there is a need. Food does not have to be transported across the world but is grown where the people are. Underground spaces offer constant climatic conditions in summer and winter. The water is reused. The Co2 footprint is extremely small compared to conventional agriculture. Aquaponics systems with aquaculture and hydroponics in a symbiotic environment function as a cycle: The water from fish tanks feeds the plants, the plants feed the people. In this way, thousands of empty tunnels and underground spaces can be used for plant cultivation without having to use up new land above ground. And without being exposed to climate change.