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Australian researchers please note: the North Sea is too shallow for gas hydrates ("Sailing too close to the wind", THES , October 31).

However, "shallow gas" and gas escape features ("pockmarks") such as the Witch's Hole are common.

Although I have heard many accounts of sea surface "boiling" caused by seabed gas emissions, the absence of documentary evidence is a problem. Also, the balance of probabilities is against a ship foundering in this way.

On the other hand, the investigation of the wreck in the Witch's Hole failed to identify any other cause for its demise. Other wrecks in pockmarks have been found in the North Sea since that survey. Perhaps inspection of these will shed light on this question.

Alan Judd
University of Newcastle upon Tyne