The white mountain of travertine is the main attraction at the Mammoth Hot Springs. This fast-changing hydrothermal area, one of the few active travertine terraces in the world, is thousands of years in the making. Marvel at the ornate step-like formation known as Minerva Terrace. Learn about thermophiles - heat-loving organisms that color hot spring pools orange, brown, yellow and green - at Canary Spring. Calculate the length of time it will take Opal Terrace to encroach on the nearby historic house. (Hint: Opal Terrace deposits 1 foot or 0.3 m of travertine a year.) Learn about pressure and time at Liberty Cap. Discover the difference between a terrace and mound on Upper Terrace Drive. Visit New Highland Terrace and witness the effects of calcium carbonate on trees. The hot spring terraces in Yellowstone's Mammoth district may not gurgle, bubble, stink or spurt but they fascinate kids nevertheless.