A petroleum seep is a place where natural liquid or gaseous hydrocarbons escape to the earth's atmosphere and surface, normally under low pressure or flow. Seeps generally occur above either terrestrial or offshore petroleum accumulation structures. The hydrocarbons may escape along geological layers, or across them through fractures and fissures in the rock, or directly from an outcrop of oil-bearing rock. The National Oceanic and Atmospheric Administration reports that "oil seeps from the ocean floor naturally release oil from subterranean reservoirs and represent the largest source [46%] of oil entering seas both in the United States and around the world. Although seeps are not without their own impacts on marine life, natural oil seeps release oil slowly over time, allowing ecosystems to adapt."