The Mohorovičić discontinuity, or simply the Moho, is the transitional boundary between the crust of the Earth and the underlying mantle located 5-10 km (3-6 mi) beneath the ocean floor and 20-90 km (10-60 mi) beneath the surface of continents. The Moho marks a transition from the basaltic rocks of the crust above it to the denser peridotitic and dunitic rocks below it. Also notably, seismic waves travelling through the Moho suddenly accelerate. Reaching the Moho has been high on the wish lists of geologists, geophysicists, and seismologists for many decades, but the difficulty of reaching such depths is immense. The most notable attempt to drill down to the Moho was the Soviet Union’s Kola Superdeep Borehole, and while it never quite reached the discontinuity, it did reveal plenty of secrets about the deeper portions of the Earth’s crust.
During the Cold War, both the United States and the Soviet Union attempted to physically penetrate the Moho (to quote one article, it was ‘the earth sciences’ answer to the space program’), hoping to reveal secrets about the internal composition of the Earth, its age, and its internal processes. Between 1958 and 1966, the US Project Mohole attempted to drill to the Moho via the ocean floor where the crust is thinnest. Drilling in extremely deep waters off the coast of Baja California, the five holes that were drilled in the project never made it more than a few hundred feet below the ocean floor before funding ran out.
The 60 m (200 ft)-tall drill rig above the Kola Superdeep Borehole, housed indoors to protect from wind exposure. Source: A. Belozeroff, http://www.panoramio.com/photo/5162451. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.
The Soviet attempt to reach the Moho was far more successful, and in time it would become the deepest hole ever dug by man; its bottom becoming the deepest artificial point on Earth. Located in the far northwestern corner of the country just a few kilometres from the Norwegian border, the Kola Superdeep Borehole was drilled beneath the tundra of the Kola beginning in 1970. This area was chosen due to its location within the Baltic Shield, which in the Kola region consisted of rocks up to 2.5 to 3.1 billion years old at the bottom. The first 7 km of drilling was uneventful as the rock was relatively homogenous (a surprise to scientists who figured that underlying basalt would be present around the 3.6 km/2.2 mi mark), but below the 7-km (4.3 mi) mark, the rock became fractured and saturated with water (hydrogen and oxygen atoms pushed out the rocks and forced together due to the intense pressure at such depths). Ever more amazingly, plankton fossils were still being discovered as deep as 6.7 km (4.2 mi).
In 1979, the Kola project’s SG-3 borehole (one of numerous boreholes which ranched off from the main hole) surpassed Oklahoma’s Bertha Rogers oil well for the title of world’s deepest hole, reaching a depth of 9 583 m (31 441 ft). The Bertha Rogers well had been drilled in 1974, with drilling having to stop once the drill reached a layer of molten sulphur so hot it melted the drill bit (no petroleum was found, by the way), but the Kola borehole was able to penetrate far beyond this mark. Eventually, extreme temperatures did put an end to drilling in 1992, by which time the depth of SG3 was measured at 12 262 m (40 230 ft), by which point the rock core samples being brought up via the borehole were dated to 2.7 billion years old. The temperature at the bottom? 180°C (356°F), a good 80°C (144 °F) warmer than had been hypothesised. The final nine years of the project saw just a few hundred metres added to the depth of the hole as drilling became exponentially difficult (at one point in 1984, a 5-km/3-mi long section of drill shaft actually broke off and had to be left in the hole, forcing the project to restart at the 7 000m/23 000 ft mark).
While the actual depth reached by the Kola borehole has not been surpassed, two wells drilled in 2008 and 2011 have since passed it in terms of total length (they were not drilled entirely horizontally, which accounts for the difference in definition). When these superdeep holes finally did come along and surpass the borehole length reached in Kola, they were not drilled for scientific purposes but for economic ones; namely, extended reach drilling for petroleum via offshore oil platforms.
The first project to surpass the Kola borehole in length was Maersk’s BD-04A well in the Al-Shaheen field off the northeast coast of Qatar in the Persian Gulf. Completed in May 2008 after a mere 36 days of drilling, the well itself was 12 289 m (40 320 ft) long, besting the Kola borehole by 27 m; the horizontal depth of 10 902 m (35 770 ft), however, was still more than a kilometre short of the Kola mark. The target? A reservoir of petroleum a mere 6 m (20 ft) thick.
Source: Hardscarf, http://commons.wikimedia.org/wiki/File:Sakhalin-1_P.PNG. Used under the terms of the GNU Free Documentation License, Version 1.2.
BD-04A was surpassed in length in January 2011 by Exxon Neftegas’ Odoptu OP-11 well, drilled in the Odoptu field off the coast of the Russian Far East island of Sakhalin as part of the company’s Sakhalin-1 project. In 60 days of drilling, OP-11 reached an ultimate length of 12 345 m (40 502 ft) and a horizontal depth of 11 475 m (37 648 ft; still nearly 800 m short of Kola). Six of the ten longest wells in the world are now found at Sakhalin-1. For now, however, the bottom of the Kola Superdeep Borehole remains the closest point that manmade objects have ever gotten to the centre of the Earth.
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