Orange Mound Spring, Thermal spring in Mammoth Hot Springs, Yellowstone National Park, US.

Orange Mound Spring is a hot spring in the Mammoth Hot Springs area of Yellowstone National Park. It stands as a raised, cone-shaped mound of travertine, the rock formed by its deposits, and is taller above the ground than the mostly flat terraces around it. Its dark orange color comes from cyanobacteria living on the travertine.

G.L. Henderson, an early assistant superintendent and guide at Yellowstone, named the spring for its dark orange color. The name refers to the orange cyanobacteria on the travertine, the rock the spring is made of. The spring's mound shape and low outflow suggest it is very old, and it has produced nearby cone-shaped springs as travertine deposits wore away.

The name Orange Mound Spring was given by G.L. Henderson, an early assistant superintendent and guide at Yellowstone. The dark orange colour that inspired the name comes from cyanobacteria growing on the travertine, and the shade shifts through the year with the nutrients the bacteria receive.

Orange Mound Spring sits in the Mammoth Hot Springs area and is reached by walking the boardwalks and trails that connect the terraces. Its flow can stop entirely on some days and run heavily on others, so the amount of water seen varies between visits. Stay on the marked paths and keep back from the travertine, which is fragile and slippery.

A network of natural fractures beneath the spring forms an underground plumbing system that channels heated rainwater and snowmelt from surrounding mountains to the surface. This hidden pipe work runs continuously and shows how geothermal activity works beneath the ground.