Have you ever wondered about how deep humankind is capable of digging into the planet? During the Cold War, both the United States and the Soviet Union tried to answer this question with vast drilling projects. These two drilling operations were opposite but strangely complimentary. The United States tried to drill through the oceanic crust to reach the Earth’s mantle through an oceanic tectonic plate (which are much thinner than continental plates, but made of dense basalt). The Soviets attempted to drill through a continental plate–which are massively thick but not nearly as dense as oceanic plates (and not underneath thousands of feet of water!). Each operation failed due to the nature of geophysical reality and to the particular weaknesses of the respective nations. In the United States, the project was abandoned because of a lack of funding caused by congressional intransigence and general scientific apathy. The Soviet project was set aside because society collapsed and the Soviet Union broke apart.
The American project was an outright attempt to drill into the Mohorovičić discontinuity, the line which separates the Earth’s crust from its mantle. The discontinuity is named after a Croatian geophysicist—and the project took its name from him as well when it came to be known as “the Mohole”. Various boreholes were sunk into the oceanic crust off the coast of Guadalupe Island, Mexico. The deepest drill hole reached 183 m (600 feet) below the sea floor—which was already beneath 3,600 meters (11,700 feet) of seawater. Yet the oceanic crust is ten kilometers (6.2 miles) thick, so the project was still far from achieving its goal. The Mohole project was plagued by mismanagement, underfunding, and incongruities between the government, scientific, and private institutions which were working together. Yet it was the first time dynamic positioning technology was used for deep sea drilling—today this technology is critical to offshore oil projects. Additionally scientist learned more about the composition of oceanic plates. Unfortunately the project was canceled in 1967.
The Soviet team began drilling began on 24 May, 1970. They chose to drill on the Kola Peninsula, which juts into the Arctic Circle between the Barents Sea and the White Sea (and is a sort of sinister eastern mirror to the great Scandinavian nations). The Soviet scientists and technicians were trying to drill through the Baltic continental crust which is estimated to be 35 kilometers (22 miles) in thickness. For decades, they worked on this project, sinking new holes as extant drillings became broken, collapsed, or unviable. The deepest they managed to drill was 12,262 metres (40,230 ft)—about a third of the distance through the continental plate. This remains the deepest drill hole in human history–although today there are a few boreholes which are longer than this (however they are not deeper–such super-long drillings are generally horizontal or diagonal for the specialized purposes of oil drilling). The Kola borehole project also produced useful and unexpected results. At the maximum depths which the drill bore reached, temperatures were much higher than expected and there was a great deal more water in the continental rock. The core samples from the drilling reached all the way through Earth’s geological history back to rocks of Archaean age (greater than 2.5 billion years old) although these were distorted by heat and pressure. Additionally the mud which came from the hole was described as boiling with hydrogen. As we dig into the underworld things get stranger and stranger! Sadly, the project was abandoned and the works are now a deserted ruin in the grim chaos of Putin’s Russia.
A perspicacious reader will note that we never actually got anywhere close to the Earth’s mantle with either of these projects. Geologists, geophysicists, and drilling engineers learned much from the attempts, but the fundamental questions about the Earth’s crust and mantle which lead to the two missions remain unanswered. All we know about the Earth’s mantle comes from the reading of various sorts of waves which pass through the Earth—not from direct observation. The only rocks we have seen from the mantle are strange xenoliths which became caught up in esoteric igneous events and traveled as tiny crystals from the mantle to the surface through volcanoes or basaltic flows. Fortunately the world’s scientists are putting together a new mission–the Integrated Ocean Drilling Program (IODP) to try again to reach the Earth’s mantle by penetrating the oceanic crust. This mission is being organized and funded mostly by the Japanese and the NSF (although there are a number of other contributing members and associate members). The Japanese in particular regard it as their premier scientific mission. Hopefully they can use today’s greatly advanced drilling technology to improve on the abortive attempts of the Americans and Soviets to pierce the crust of the planet.