By Cara Froedge

Unlike Wyoming’s steaming geysers, Periodic Spring near Afton, Wyo., gushes an on-off torrent that’s a chilly 40 degrees. What makes it start and stop on a 30-minute cycle? No one is sure. Only in late summer and winter is the otherwise steady flow intermittent.

Hydrologists theorize that the spring originates in an underground reservoir fed by snowmelt and rainwater seeping through the Salt River Range. When the lake fills, a natural siphon draws water to the surface, where it pours into Swift Creek—except in seasons when the surface flow is light.

To visit, turn east off Highway 89 in Afton onto Second Avenue and drive five miles upstream on Swift Creek Road. A steep trail ends at the spring. (307) 885-9831.