The park’s hydrothermal systems are an enigma. The iconic geyser erupts every 90 minutes, on average, like clockwork and lasts for up to five minutes. Scientists still don’t understand how and why Old Faithful erupts at varying intervals. Research has shown that a large egg-shaped chamber is connected to the mouth of Old Faithful by something like a pipe. Water levels rise after every eruption and send steam bubbles into the conduit—which creates a “bubble trap” that leads to the eventual steam explosion.