Scientists have shown that Parisian street gutters are oases of microscopic life, home to microalgae, fungi, sponges, and mollusks. Grouped into communities, these microorganisms may help clean rainwater and urban waste by decomposing solid debris and pollutants. A deeper understanding of the role and composition of these communities could help elucidate the services rendered by gutter ecosystems. The researchers’ findings are the first to reveal the unsuspected biodiversity of microscopic life in Paris city streets.