Yellowstone, the original.

Field guide ◆ Yellowstone

Plan the trip.

Yellowstone is the park that started national parks. Set aside in 1872 — before Yosemite, before Glacier, before any of the others — its 3,500 square miles cover Wyoming, Montana, and Idaho, sit on top of the largest active volcanic system in North America, and contain more than half the world's geysers.

For a first visit, plan a 4-day loop. Day 1: Mammoth Hot Springs, drive south to Norris Geyser Basin, sleep at Madison. Day 2: Old Faithful and the Upper Geyser Basin (stay all morning, the lesser-known geysers are the magic). Day 3: Lamar Valley at sunrise (bison and wolves) and Hayden Valley at sunset. Day 4: Yellowstone Lake and the South Entrance to Grand Teton.

Best window: late May to early September. June is wildflowers and fewer crowds; July-August is full-on; September is gold and grizzly-active. Avoid winter unless you're committed — most roads close.