A carbon sink is anything, natural or otherwise, that accumulates and stores some carbon-containing chemical compound for an indefinite period and thereby removes carbon dioxide () from the atmosphere. These sinks form an important part of the natural carbon cycle. An overarching term is carbon pool, which is all the places where carbon can be (the atmosphere, oceans, soil, plants, and so forth). A carbon sink is a type of carbon pool that has the capability to take up more carbon from the atmosphere than it releases. Globally, the two most important carbon sinks are vegetation and the ocean. Soil is an important carbon storage medium. Much of the organic carbon retained in the soil of agricultural areas has been depleted due to intensive farming. "Blue carbon" designates carbon that is fixed via the ocean ecosystems. Coastal blue carbon includes mangroves, salt marshes and seagrasses which make up a majority of ocean plant life and store large quantities of carbon. Deep blue carbon is located in the high seas beyond national jurisdictions and includes carbon contained in "continental shelf waters, deep-sea waters and the sea floor beneath them. As a main carbon sink, the ocean removes excess greenhouse gas emissions such as heat and energy." Many efforts are being made to enhance natural carbon sinks, mainly soils and forests, to mitigate climate change. These efforts counter historical trends caused by practices like deforestation and industrial agriculture which depleted natural carbon sinks; land use, land-use change, and forestry historically have been important human contributions to climate change. In addition to enhancing natural processes, investments in artificial sequestration initiatives are underway to store carbon in building materials or deep underground. In the context of climate change and in particular mitigation, a sink is defined as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere".