In The Smithsonian, Ker Than tells of geologists following a hunch about odd-looking stalagmites and discovering a way to study ancient earthquakes. The drip of water from a cave ceiling to a floor creates a low-tech record of seismic activity, writes Ker. He has lots more in The Smithsonian, @kerthan, and on his website. usgs-great_onyx_cave_kentuckyimage: USGS