From http://www.networkworld.com/news/2006/090706-pc-hard-disk-system-warns.html:

As part of their operation, hard disks measure vibrations in order to keep the read-write head of the disk on track.  These measurements can be read from some hard disks.  The Tsunami Harddisk Detector captures this vibration data and shares it with computers in other locations connected via a peer-to-peer network to determine whether an earth tremor is occurring.

Neat.

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