Nap-of-the-earth (abbreviated NOE) is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment. During NOE flight, geographical features are used as cover, exploiting valleys and folds in the terrain by flying in, rather than over, them. This keeps below enemy radar coverage, avoiding "skylining". Other terms are also used, including "ground-hugging", "terrain masking", or "flying under the radar". The older term, commonly used for fixed-wing aircraft practicing this flight tactic in the World War II era, was "hedgehopping".