Yellowstone’s volcano heats water deep underground. Under great pressure, the water percolates upward through buried limestone, dissolving a mineral called calcium carbonate. Above ground, the water begins to cool and evaporate. Gasses are released and water pressure decreases. Orange Spring Mound gradually grows as the water flows over it, depositing calcium carbonate, or travertine. Hot water flows over the mound after bubbling through several vents along the top. Heat-dwelling bacteria and algae grow abundantly in Orange Spring Mound’s water, creating tapestries of "living color."