Geophone - SM-24, with Insulating Disc

The SM-24 geophone element is designed to offer the highest performance in seismic exploration. A geophone works by translating ground movement into voltage, which can easily be read by a microcontroller. Low distortion, combined with excellent specifications, provide high-fidelity data.

Basically, it's a super low frequency microphone for the ground. Perfect for all of your 2-D & 3-D seismic exploration needs with bandwidth from 10 Hz up to 240 Hz.

More information is available on the website of SparkFun.