While the crowds swarm around Old Faithful to wait for its next eruption, a little pool just north of Yellowstone National Park's most famous geyser is quietly showing off its own unique activity, also at more-or-less regular showtimes. Instead of erupting in a towering geyser, though, Doublet Pool cranks up the bass every 20 to 30 minutes by thumping. The water vibrates and the ground shakes.