Humans have only managed to drill into the Earth's crust, which is the outermost layer of the planet. The deepest hole ever drilled by humans is the Kola Superdeep Borehole, which reached a depth of 12,262 meters (40,230 feet) into the Earth's crust in 1989. This borehole is located in Russia and was part of a scientific project to study the Earth's interior. The Earth's crust is made up of solid rock and is generally between 5 and 70 kilometers (3 and 43 miles) thick. The next layer of the Earth, the mantle, is made up of hot, semi-solid rock and is much thicker than the crust, extending down to a depth of about 2,900 kilometers (1,800 miles). Humans have not yet been able to drill into the mantle due to the extreme heat and pressure at those depths.