If only humanmade plumbing were as reliable as Old Faithful’s. Every 90 minutes or so, Yellowstone National Park’s iconic geyser (pictured) spews water and steam 40 meters high, on a schedule so precise that tourists know when to gather to watch the show. Now, geophysicists have caught a glimpse of the subsurface reservoir that supplies the clockwork gusher. Most subsurface imaging relies on seismic waves, often generated by small explosions or by “thumper” trucks—measures far too disruptive for a national park. So instead, researchers relied on natural vibrations: They listened for seismic waves from the gurgling and popping of water and steam percolating through the ground near Old Faithful, using 133 seismographs spread across a square kilometer. The criss-crossing signals revealed a region of porous rock 200 meters across and 50 meters thick, just to the west of the geyser, the team reports this month in Geophysical Research Letters. This underground tank holds hundreds of millions of liters of magma-heated water, the researchers believe. Fed by that mighty plumbing system, Old Faithful is in no danger of running dry.