Like probes sent into outer space, scientific drilling is a technology used to obtain samples from places that people cannot reach.  Human beings have descended as deep as 2,080 m (6,822 ft) in Voronya Cave, the world's deepest known cave, located in the Caucasus mountains of the country of Georgia. Gold miners in South Africa regularly go deeper than 3,400 m, but no human has ever descended to greater depths than this below the Earth's solid surface. As depth increases into the Earth, temperature and pressure rise. Temperatures in the crust increase about 15°C per kilometer, making it impossible for humans to exist at depths greater than several kilometers, even if it was somehow possible to keep shafts open in spite of the tremendous pressure.[1]

The Kola Superdeep Borehole on the Kola peninsula of Russia reached 12,262 metres (40,230ft) and is the deepest penetration of the Earth's solid surface.  The German Continental Deep Drilling Program at 9.1 kilometres (5.7mi) has shown the earth crust to be mostly porous. Drillings as deep as 2.1 kilometres (1.3mi) into the seafloor were achieved at DSDP/ODP/IODP Hole 504B. Because the continental crust is about 45 km thick on average, whereas oceanic crust is 6-7 km thick, deep drillings have penetrated only the upper 25-30% of both crusts.