A novel mechanism for long-term air retention under water is found in the sophisticated surface design of the water fern Salvinia. Its floating leaves are evenly covered with complex hydrophobic hairs retaining a layer of air when submerged under water. Surprisingly the terminal cells of the hairs are hydrophilic. These hydrophilic patches stabilize the air layer by pinning the air water interface. This "Salvinia Effect" provides an innovative concept to develop biomimetic surfaces with long-term air-retention capabilities for under water applications.