Two thirds of the world’s geysers are located here at Yellowstone National Park, each with its own unique characteristics. Shown here is the Great Fountain Geyser, which erupts every 9 to 15 hours with sprays that reach 75 feet to over 220 feet in the air. More than just a spectacle, these geysers are evidence of the volcanic activity simmering beneath the surface. Yellowstone’s last major volcanic eruption occurred about 630,000 years ago, and the potential for future activity is the subject of much curiosity—and speculation.