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How deep is the Mariana Trench? Oceanography!

It may come as a surprise to you to learn that we don't know precisely how deep the Mariana Trench is. But it is not so strange, if you think a bit more about it. Have you ever wondered how we would measure this depth? You can't exactly use a yardstick, the way you measure fabric for a dress or the length of a wall for wallpaper.

Instead, scientists send sound waves through the water and study their reflections (their echoes). This may give results, but those results need to be interpreted, translated into meaningful data. All sorts of calculations need to be made for pressure effects and composition of the water (how the molecules interact and transmit sound).

This is where opinions come in because not all scientists have to agree on how to do that. This is also where technology comes in. As you can imagine, the development and improvement of new technologies often leads to more precise measurements and the adjustment of older data.

Still, we have a pretty good idea of its depth. If you scan the literature, you will find various numbers for the depth of the Mariana Trench:

• In 1957, the R/V Vitiaz measured a depth of 10.990 m. This was later corrected to 11.034 m.
• The bathyscaphe Trieste determined the depth as 10.911 m, in 1960.
• A survey carried out in 1984 concluded that the depth was 10.924 m at 11°22.4'N, 142°35.5'E
• The R/V Hakuho Maru recorded a depth of 10.550 m in 1992 and that was later corrected to 10.933 m.
• Kaiko recorded depths of 10.898 m during dives in 1995 and 1996.
• A study decribed in a 2003 paper measured a depth of 10.744 m at 11°22.927'N, 142°26.258'E, but the authors of this article say that their calculation was not as accurate as that of the 1984 survey.

The Mariana Trench is the type of trench that usually is rather deep. But it is a little bit deeper than expected and its deepest part is not where you would expect it to be. How come?

In 2003, the journal "Earth and Planetary Science Letters" published an article by six authors, entitled "Why is the Challenger Deep so deep?" The Challenger Deep is the deepest part - as far as we know - of the Mariana Trench. (There are several deeps in the Mariana Trench.)

When I spotted this paper, I told my boss "I bet quite a few folks would like to hear about that." She is more into chemistry, but she bought it for me. I later saw that New Scientist had also written about it (June 14, 2003, page 16).

In case you wonder, a (scientific) journal is a magazine in which scientists write articles to tell each other what kind of work they have done. They do not get paid for those articles because writing is part of their job. Sometimes, they even have to pay, for example if they want to include color images.

Abstracts of scientific articles - also called papers - are available on the web, for instance, here (at Ingenta - a large literature database) and here (at Elsevier, the publishing company of the article). I cannot show the article or post these abstracts here, as I would be breaking copyright law if I did. More and more articles are freely available on the web these days.

What I can do without breaking the law, is talk about this article - "Why is the Challenger Deep so deep?" and explain it somewhat.