While the Earth's mantle is much larger, it is much too hot and under too much pressure to be accessible to humans. The deepest mine is the Mponeng gold mine in South Africa, reaching about 2.4 miles (3.9 kilometers) below the surface. The deepest drill hole, the Kola Superdeep Borehole in Russia, reached a depth of 7.5 miles (12.3 kilometers). Both of these are still far from reaching the boundary between the crust and the mantle, which is estimated to be around 18 to 31 miles (30 to 50 kilometers) deep.