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 I studied oceanography many years ago at university and recall a lecturer claiming that submarines in the second world war could pass undetected through the entrance to the Mediterranean.

 He claimed that the halocline caused by the different salinities of the Atlantic and Mediterranean seas would be enough to deflect sonar thus making the submarine invisible. Has anyone seen evidence to support this?

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    $\begingroup$ it looks like there is a lot of information about this on the net,this is one of them en.wikipedia.org/wiki/Thermocline it is mentioned in the oceans part of this article. $\endgroup$ – trond hansen Sep 11 at 10:50
  • $\begingroup$ @trondhansen Thanks for the link. I'd obviously miss-remembered this. I thought it would be the halocline rather than the thermocline that would cause this! Do you want to update your comment as an answer? $\endgroup$ – MiguelH Sep 11 at 10:54
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Because of the limitations of wartime sonar, U-boats could sometimes pass undetected into the Mediterranean through the Straits of Gibraltar, but it was always risky and avoiding detection couldn't be guaranteed.

There were many things that limited the efficiency of sonar (or ASDIC as the British called it). It is true that sea water has a tendency to separate itself into layers which sonar finds difficult to penetrate. Sometimes this layering was based on differences in salinity, sometimes on differences in temperature, and sometimes on intervening sea currents. Even ships wakes could deflect the sonar beam, so sometimes powerful curving wakes were deliberately made by the U-boat to shake off a pursuing warship.

The range of active sonar under good conditions was normally a few miles, but that could be reduced if there was a lot of background noise. The Gibraltar straits are about 12 miles wide. Whales and schools of fish could also send back false echoes which would fool all but the most experienced sonar operators.

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    $\begingroup$ Thanks for this. It does seem to coincide with what I remember from the lecture. Do you have a link to any source for more information? $\endgroup$ – MiguelH Sep 11 at 11:16

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