The transition zone between the Earth's upper and lower mantle contains considerable quantities of water, according to an international study involving the Institute for Geosciences at Goethe University in Frankfurt. The German-Italian-American research team analyzed a rare diamond formed 660 meters below the Earth's surface using techniques including Raman spectroscopy and FTIR spectrometry. The study confirmed something that for a long time was only a theory, namely that ocean water accompanies subducting slabs and thus enters the transition zone. This means that our planet's water cycle includes the Earth's interior.