A microscopic pocket sealed inside a diamond only 3 millimeters long may record how water travels from Earth’s surface into the planet’s deep interior. Below the upper mantle lies the lower mantle, where similar chemical ingredients form different crystal structures under greater heat and pressure. Super-deep diamonds were first identified near Juína in the late 1980s and have contributed to numerous discoveries about Earth’s interior. Some researchers remain cautious because shock compression does not perfectly reproduce conditions in either the upper or lower mantle. The lower mantle remains more difficult to explain because most of its minerals can store very little water.