or a seismologist's sigh.
In grad school I played around with an exploration seismograph. Twelve geophones, a computer, and a sledgehammer. We could easily see trucks on a nearby road and even footfalls in the realtime graphs.
It reminds me of the time I was mapping a small mine that was less than a half mile from a train track. A weird experience, being in the silent dark a couple of hundred feet below the sunlight, a feeble light strapped to my head with the light cone shifting with my glance in the stale, particle filled air; bats fluttering about while I was standing in a copper blue puddle with a hundred feet of open stoping above and the walls themselves vibrating with the passing of several thousand tons of freight.