Topless Mark Twain.
This is me, about 100 meters underground in a dolostone mine, about to hit a 15 m x 15 m pillar of rock with a hammer. The little blue thing to the right is one of 24 geophones coupled into the sides of the pillar. They detect and record the seismic waves created by my hammer blow. I use these seismic waves to create an image of the interior of the pillar. Pretty cool, right?