By 2050, as the planet's resources will no longer be sufficient to meet everyone's needs, Earth will have nearly 10 billion inhabitants. Pollution, depletion of fossil fuels, climate change... it is becoming necessary to change our energy habits. From Iceland to the Canary Islands, through the Netherlands and Paris, this episode of a four-part series showcases various engineering and research projects. Meeting with professionals dedicated to finding sustainable solutions, it reveals how to better understand marine and aquatic environments in order to exploit them respectfully.