there is a conversion to a more dense mineral structure, the seismic velocity rises abruptly and creates a discontinuity. At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite. Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This has led to the hypothesis that the transition zone may host a large quantity of water. In Earth's interior, olivine occurs in the upper mantle at depths less than , and ringwoodite is inferred within the transition zone from about depth. Seismic activity discontinuities at about , , and depth have been attributed to phase changes involving olivine and its polymorphs. At the base of the transition zone, ringwoodite decomposes into bridgmanite (formerly called magnesium silicate perovskite), and ferropericlase. Garnet also becomes unstable at or slightly below the base of the transition zone. Kimberlites explode from the earth's interior and sometimes carry rock fragments. Some of these xenolithic fragments are diamonds that can only come from the higher pressures below the crust. The rocks that come with this are ultramafic nodules and peridotite. Chemical composition The composition seems to be very similar to the crust. One difference is that rocks and minerals of the mantle tend to have more magnesium and less silicon and aluminum than the crust. The first four most abundant elements in the upper mantle are oxygen, magnesium, silicon, and iron. Exploration Exploration of the mantle is generally conducted at the seabed rather than on land because of the oceanic crust's relative thinness as compared to the significantly thicker continental crust. The first attempt at mantle exploration, known as Project Mohole, was abandoned in 1966 after repeated failures and cost overruns. The deepest penetration was approximately . In 2005 an oceanic borehole reached below the seafloor from the ocean drilling vessel JOIDES Resolution. On 5 March 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, midway between the Cape Verde Islands and the Caribbean Sea. The exposed site lies approximately beneath the ocean surface and covers thousands of square kilometers. The Chikyu Hakken mission attempted to use the Japanese vessel Chikyū to drill up to below the seabed. On 27 April 2012, Chikyū drilled to a depth of below sea