Hidden in an abandoned drill site among rotting wood and sheets of scrap metal — remains of the derrick and housing that once stood in Russia — sits a small, unassuming, heavy duty maintenance hole cover secured into place with a dozen large, rusting bolts. Underneath — and virtually unseeable from ground level — at just 9 inches (23 centimeters) in diameter, is the world’s deepest borehole.
The Kola Superdeep Borehole runs about 40,230 feet (12,262 meters) or 7.6 miles (12.2 kilometers) into Earth’s surface. For perspective, the hole’s depth is the height of Mount Everest and Mount Fuji placed on top of one another. It’s also deeper than the deepest point of the ocean, the Mariana Trench, which lays at a depth of 11,034 meters (36,201 feet) below sea level.
For perspective, Earth’s outermost layer — the ground we stand on — called the continental crust, is about 25 miles (40 kilometers) thick.
The next layer, the mantle, continues for another 1,800 miles (2,896 kilometers). The outer core extends about 1,400 miles (2,250 kilometers) before reaching Earth’s inner core, a hot, dense, mostly iron ball with a radius of about 758 miles (1,220 kilometers). From where you are standing, Earth’s core is about 1,802 miles (2,900 kilometers) below your feet.
So, while Kola is an impressively deep borehole, it is surprisingly shallow compared to Earth’s depth. In total, Kola only penetrates about a third of Earth’s crust and 0.2 percent of the entire distance to the center of Earth.
It also took a while. Years in fact. Drilling at Kola began May 24, 1970. The goal was to go as far as possible, which scientists at the time expected to be about 9.3 miles (15 kilometers). By 1979, the project had broken all world records for man-made holes when it surpassed about 6 miles (9.5 kilometers).
In 1989, drilling reached a depth of 40,230 feet (12,262 meters) vertically below Earth’s surface. It is the deepest point ever reached. That’s when temperatures in the well increased from the expected 212 degrees Fahrenheit (100 degrees Celsius) to 356 degrees Fahrenheit (180 degrees Celsius). (More on this in a minute.)