Far beneath Earth’s surface oceans lies a hidden world of water, often referred to as a “sea under the sea.” This concept encompasses significant quantities of water found deep within our planet’s interior, extending beyond mere underground rivers or lakes. Scientists have uncovered evidence of vast reservoirs of water, existing in various forms, far removed from the oceans we navigate. These subterranean waters represent a substantial component of Earth’s overall water budget, challenging previous understandings of our planet’s composition and dynamics.
Water Trapped in Earth’s Mantle
A considerable amount of water is chemically bound within minerals deep inside Earth’s mantle, particularly within the transition zone. This region, located between approximately 410 and 660 kilometers (250 to 410 miles) below the surface, acts like a sponge. It absorbs and stores water molecules within the crystal structures of high-pressure minerals such as ringwoodite and wadsleyite. Unlike surface liquid water, this water is not free-flowing; it is incorporated into the mineral lattice, often as hydroxyl (OH-) ions.
Scientists estimate that the transition zone alone could hold a volume of water equivalent to, or even several times, that of all Earth’s surface oceans combined. For instance, if just 1% of the water stored in ringwoodite within this zone were released, it could triple the volume of our current oceans. This discovery, confirmed by analyzing rare diamonds containing ringwoodite inclusions, has revolutionized our understanding of Earth’s internal structure and its water cycle. The presence of this deep water reservoir indicates a much more complex internal water system than previously imagined.
Subterranean Water Reservoirs
Beyond the mineral-bound water in the mantle, Earth also harbors large bodies of liquid water within its crust or upper mantle, separate from the global ocean system. These subterranean reservoirs include vast deep aquifers, which are underground layers of water-bearing permeable rock, and subglacial lakes. Subglacial lakes are found beneath glaciers and ice sheets, such as those in Antarctica and Greenland. Over 400 subglacial lakes have been identified in Antarctica alone, holding an estimated 10,000 cubic kilometers of water, representing about 15% of Earth’s liquid freshwater.
These liquid reservoirs differ significantly from the chemically bound water in the mantle. Water exists in a liquid state, even under immense pressure or at temperatures below freezing, often due to geothermal heating, the pressure of overlying ice, or high salt content. For example, in subglacial lakes, the pressure from the overlying glacier lowers the melting point of water, allowing it to remain liquid. Deep groundwater systems can also contain hypersaline brines that remain liquid at very low temperatures.
Implications for Earth’s Systems
The presence of this deep water, both chemically bound in mantle minerals and as liquid subterranean reservoirs, holds significant implications for various Earth processes. Water influences plate tectonics, acting as a lubricant in subduction zones where oceanic plates descend into the mantle. This affects the movement of tectonic plates and contributes to earthquake generation. The release of water from subducting plates also lowers the melting point of surrounding rocks, leading to magma formation and subsequent volcanic activity.
Deep water also plays a role in the deep carbon cycle, influencing how carbon is transported into and out of Earth’s interior. The movement of volatiles like water, sulfur, and carbon dioxide through the deep Earth impacts continent formation and the distribution of mineral resources. A stable deep water cycle may also contribute to Earth’s long-term habitability by regulating surface temperatures and maintaining liquid water on the surface over billions of years.
Methods of Discovery
Scientists employ several methods to detect and study this hidden water deep within Earth. Seismic tomography is a primary technique, involving analysis of how seismic waves generated by earthquakes travel through the Earth’s interior. Variations in the speed and behavior of these waves provide clues about the composition and properties of the rocks they pass through, including water presence. Researchers infer water content by observing changes in seismic velocity, as water affects how sound waves move through materials.
Laboratory experiments also play a role, as scientists recreate the extreme pressure and temperature conditions of the deep mantle. By subjecting minerals like olivine and ringwoodite to these conditions, they observe how much water these minerals can hold and how their properties change. The analysis of rare mineral samples, particularly diamonds containing tiny inclusions from the deep mantle, provides direct evidence. These diamonds can trap microscopic pockets of minerals formed at great depths, preserving clues about the water content and chemical environment of the deep Earth.