Blue Heart Springs is a natural thermal spring on the Snake River near Thousand Springs State Park in Hagerman, Idaho, known for its vivid blue water and heart-shaped cove. It is the thirteenth largest freshwater spring in North America and is accessible only by water. Its setting reflects ancient geology: the cove is believed to have formed about 15,000 years ago when a whirlpool during the Bonneville Flood cut into the Snake River Aquifer. Water percolates through sand, likely spending about 100 to 150 years underground before emerging. A steady 58°F temperature and enclosing melon gravel walls give the pool its distinctive character.