This is Silex Spring, on Yellowstone National Park's Fountain Paint Pot Trail.
According to the National Park Service, "Hot springs are the most common geothermal feature in Yellowstone. Their underground plumbing consists of unconstricted chambers of varying sizes. Water, which has been superheated far underground, rises along tiny cracks, flows into larger channels and eventually reaches the earth's surface.
"Unlike geysers, water convection is continuous within hot spring pools. Hot surface water sinks while hotter, superheated water continues to rise. This circulation keeps water below the temperature needed to begin the chain reaction leading to an eruption."