The Beijing Anomaly is a seismic wave

Seismic wave

Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...

 anomaly in the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's mantle

Mantle (geology)

The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

, from ~700–1400 km depth, characterized by relatively high attenuation and decreased sound speed. It is centered beneath northern China, just northwest of Beijing.

The seismic properties of the anomaly are similar to those of the asthenosphere

Asthenosphere

The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

, which contains water absorbed from subducting slabs

Subduction

In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...

. The discoverers of the anomaly, Jesse Lawrence of Scripps

Scripps Institution of Oceanography

Scripps Institution of Oceanography in La Jolla, California, is one of the oldest and largest centers for ocean and earth science research, graduate training, and public service in the world...

 and Michael E. Wysession

Michael E. Wysession

Michael E. Wysession is currently an Associate Professor of Earth and Planetary Sciences at Washington University in St. Louis, and author of numerous science textbooks with Pearson Education and Prentice Hall. He earned his Sc.B. from Brown University in 1980 and his Ph.D...

 of Washington University, propose that it has a similar water content, from subducting lithosphere

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 from the Pacific plate

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....

. This would imply that the amount of water in this region of the mantle is comparable to that in the Arctic Ocean

Arctic Ocean

The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

 (although the water content is no more than 0.1% by volume). Other possible causes of the anomaly include small-scale chemical heterogeity and abnormally small particle size.

Popular culture

• The anomaly, and other postulated "underground oceans", are key to the plot of Stephen Baxter
  Stephen Baxter
  Stephen Baxter is a prolific British hard science fiction author. He has degrees in mathematics and engineering.- Writing style :...
  
  's 2008 near-future disaster novel Flood
  Flood (Baxter novel)
  Published in 2008, Flood is the work of hard science fiction by English author Stephen Baxter. It describes a near future world where deep submarine seismic activity leads to seabed fragmentation, and the opening of deep subterranean reservoirs of water. Human civilization is almost destroyed by...
  
  .