Mammoth Hot Springs

Mammoth Hot Springs is the place where Yellowstone builds in stone. On a hillside at the park's northern edge, hot water rises through limestone, dissolves it, and on reaching the air drops it again as travertine — more than two tons a day. The result is a staircase of terraces, pools and mounds that grows, dries and shifts from year to year: white where the flow has stopped, and tinted orange, brown, red and green where heat-loving microbes live in the warm water. Terrace Mountain here is the largest known carbonate-depositing spring in the world.

The water comes from a long way off. Heat from the same magma system that feeds the rest of Yellowstone warms water at Norris Geyser Basin, which then travels north underground along a fault through limestone, cooling on the way until it emerges at no more than about 73 °C. Because the vents keep moving, the scenery never settles: Minerva Terrace, the most famous of the formations, has gone dry after the flow shifted. The names read like a classical dictionary — Cleopatra, Minerva, Aphrodite, Cupid — alongside Liberty Cap, an extinct spring cone standing alone like a stone thumb, and Orange Spring Mound.

The hill also carries history. Fort Yellowstone, an army post whose construction began in 1891, stands on an old terrace, and when construction began there were fears the hollow ground would not bear the buildings; fenced sinkholes can still be seen on the parade ground. Traces of the last glaciation are everywhere too, from glacial till on top of Terrace Mountain to thermal kames such as Capitol Hill. Elk are often seen standing on the terraces themselves, pale animals on pale stone.

Getting there

Mammoth Hot Springs lies about 5 miles (8 km) inside the park's North Entrance at Gardiner, Montana. The practical airport is Bozeman Yellowstone International (BZN), about 82 miles (130 km) by road, roughly two hours' drive via Gardiner.