We report evidence of dense, ordered nanodomains in single-component fluid lipid bilayers. Our atomic-scale molecular dynamics simulations suggest that the area available to a lipid acyl chain exhibits large fluctuations, resulting in denser and sparser domains. The sizes of the dense domains can be up to ~10¿¿nm, and their lifetimes are of the order of ~10¿¿ns. In addition, our simulations suggest that domains of lipids with highly ordered acyl chains form predominantly within the dense regions, their sizes ranging from a few chains up to a few nanometers, and with lifetimes between ~10¿¿ps–10¿¿ns. These observations shed light on the origin of experimentally observed fluctuations, as well as on the mechanisms of phase transitions in lipid membranes.