A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool. Cold does not mean that the temperature of the seepage is lower than that of the surrounding sea water. On the contrary, its temperature is often slightly higher. The "cold" is relative to the very warm (at least 60 °C or 140 °F) conditions of a hydrothermal vent. Cold seeps constitute a biome supporting several endemic species.
Cold seeps develop unique topography over time, where reactions between methane and seawater create carbonate rock formations and reefs. These reactions may also be dependent on bacterial activity. Ikaite, a hydrous calcium carbonate, can be associated with oxidizing methane at cold seeps.