The transition zone between the Earth's upper and lower mantle contains considerable quantities of water, according to an international study. The 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.