SPECULATIVE SCIENCE

What is the deepest man has penetrated below the earth's surface, and what proportion does this constitute of the distance to the core?

Tony Stern, London W2

  • The deepest is 3,777 metres at the Western Deep Levels gold mine in the Transvaal, South Africa. The earth is not spherical and the best approximation is an ellipse rotated about its minor axis, so distance to the centre of the earth varies with latitude. The earth's equatorial radius is 6,378km; assuming this is reasonably accurate for the Western Deep Levels, the depth represents little more than 0.05% of the distance to the centre of the earth, although a latitude correction should be made. The depth to the core, as opposed to the centre of the earth, is approximately 2,898km, so the mine depth is marginally over 0.1% of this figure.

    Dave Haynes, Lancaster

  • Some of the earlier answers have been misleading. Project Mohole was funded by the US government in 1957, stoked by the belief that the USSR was planning deep drilling. The pilot project succeeded in drilling 197m -not 197km, as Philip Sills stated (Notes & Queries, March 30) - into the bed of the Pacific ocean from a drilling barge on the surface nearly 3km above. The project was abandoned in 1966 when unsavoury financial links between the Texan oil-drilling company running the project and the Democratic Party came to light.

    The USSR had indeed planned since about 1960 to drill deep holes onshore. The Superdeep Number 3 in Kola, Russia, remains the world's deepest hole into the earth, at 12,261m. It took from 1970 to 1984 to get this far. One of the scientists in charge told me that they were keen to license their world-leading drilling technology to the west for hard currency. But the government decreed that since the west wanted the knowledge, then it should be classified as a state secret - so no deal. The project was put on hold due to lack of funds, and the Russians' capability to drill to 15,000m will probably never be tested. However, the results disproved outdated concepts held by influential conservative USSR geologists, and the plate tectonic paradigm came to be officially accepted. The German deep drill hole, the KTB in Bavaria, was planned to drill to 10,000m, but drilling was abandoned in 1994 at a depth of 9,100m, as the temperature of 280C (536F) at the bottom turned out to be hotter than expected. Every time the drill-bit was withdrawn for replacement, the lower portion of the hole closed up by plastic flow of the rocks. Thus it will probably never be possible to drill much deeper unless you choose a region of cold crust, as the Russians did.

    Prof David Smythe, Glasgow

  • Jeremiah 31:37

    Tim D, Michigan USA

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