level, setting a new world record for deep-sea drilling. This record has since been surpassed by the ill-fated Deepwater Horizon mobile offshore drilling unit, operating on the Tiber prospect in the Mississippi Canyon Field, United States Gulf of Mexico, when it achieved a world record for total length for a vertical drilling string of 10,062 m (33,011 ft). The previous record was held by the U.S. vessel Glomar Challenger, which in 1978 drilled to 7,049.5 meters (23,130 feet) below sea level in the Mariana Trench. On 6 September 2012, Scientific deep-sea drilling vessel Chikyū set a new world record by drilling down and obtaining rock samples from deeper than below the seafloor off the Shimokita Peninsula of Japan in the northwest Pacific Ocean. A novel method of exploring the uppermost few hundred kilometers of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe that melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks. The probe consists of an outer sphere of tungsten about in diameter with a cobalt-60 interior acting as a radioactive heat source. This should take half a year to reach the oceanic Moho. Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago. References Earth's mantle