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Union University Department of Physics

The Science Guys

Science Guys > April 2003

April 2003

What is the deepest hole ever dug into the Earth?

We would all like to know what is inside the Earth. Digging a hole straight down would seem to be the most effective way to study the Earth’s interior. Many holes have been dug for both research and commercial purposes. However, by digging, we have only scratched the surface of our Earth - literally. If the Earth were an egg we haven’t even succeeded in boring through the shell. Actually, the Earth does have a shell, called the crust. The Earth’s crust is broken into many smaller plates that slide very slowly over a more mobile or ’plastic’ material called the asthenosphere.

The deepest hole within the U.S. is the Bertha Rogers gas well in Oklahoma at 32,000 feet (6 miles) deep. The well was halted because it struck molten sulfur. Perhaps the most well-known effort to pierce the Earth is Project Mohole (began in 1961), which was an attempt to drill through the Earth’s crust in the Pacific Ocean off the coast of Mexico where the crust is shallow. In 1966 funds ran out and the project closed. The goal was to reach a discontinuity between the upper mantle and the Earth’s crust called the Mohorovicic discontinuity - commonly termed the "Moho." The project fell quite short of the Moho, reaching only 601ft below the sea floor in 12,000 feet of water. Where they were digging, the Moho is 16,000 ft deep, so the team fell far short of their goal. However, valuable core samples were obtained and a lot was learned about deep sea drilling.

The deepest hole by far is one on the Kola Peninsula in Russia near Murmansk, referred to as the "Kola well." It was drilled for research purposes beginning in 1970. After five years, the Kola well had reached 7km (about 23,000ft). Work continued until the project was abandoned in 1989 because the drill became stuck in rock at a little over 12km (almost 40,000ft or 8 miles) deep. That is the current record for a depth reached by humans. The Kola’s well depth is comparable with the distance across Jackson. The project cost over $100 million, which is about $2500 per foot. That is expensive digging! Given the technology and funds, geologists would like to try to go deeper for core samples, but digging such holes requires much patience, money, technology, and luck. Much information comes from such holes; for example, the bottom of this hole was about 370°F (190°C).

The structure of the deep Earth is studied today by means that are more indirect. Perhaps the most effective method has been from studying earthquake or seismic waves as they move from one sensing station to another. These natural waves allow us to see inside the Earth as they react to various layers, much as x-rays or MRI’s allow us to view inside the human body.