Colter's Hell is the name given to a stretch of the Shoshone River near Cody where hot springs and gas seeps were observed by mountain man John Colter, who passed through in 1808 on his way back from the first American expedition to the Yellowstone region. The geothermal activity here is minor compared to Yellowstone's geyser basins, but the site has historical significance as one of the earliest documented encounters between Euro-American explorers and the geothermal character of the Greater Yellowstone region.
The accessible area is along the river corridor west of Cody. The visible features — mineral deposits, sulphurous seeps — are subtle rather than dramatic, and visiting requires some interest in historical context to be worthwhile. If you are driving the Wapiti Valley route to Yellowstone, it is worth a brief stop to stand at the spot and think about what Colter was trying to explain to a disbelieving audience back east.