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  &#8220;One of the common ways was to make spectrographs &#8211; looking at how the spectrum of sound waves developed over time &#8211; and in this we saw the unique signal.&#8221;
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<p>
  The two researchers describe the unique signal found on spectrograph plots recorded by Indian Ocean hydrophones as a &#8220;chirp&#8221;.
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<p>
  What it means is that low-frequency vibrations are arriving before those of higher frequencies, producing a distinctive upward curving slope.
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<p>
  &#8220;In this frequency range &#8211; and these are very low frequencies, well below 1Hz &#8211; this is a unique signal,&#8221; said Dr Bowman.
</p>

<p>
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