Detecting the imminent eruption of a geyser involves observing a combination of visual and auditory "pre-play" signals. For many geysers, the first sign is overflowing water; as pressure builds in the underground plumbing, the water level in the vent rises until it begins to spill over the rim. You might also see surging or splashing, where small bursts of water are thrown a few feet into the air as steam bubbles begin to escape. Another reliable indicator is a change in the steam column; the steam may become thicker, more frequent, or begin to "chuff" like a steam engine. Auditory cues include deep, subterranean rumbling or thumping sounds, which are caused by steam bubbles collapsing (cavitation) as they move through the narrower parts of the geyser's throat. In famous locations like Yellowstone, "Old Faithful" is predictable based on its previous eruption's duration, but for most "wild" geysers, these physical surges and overflows are the only warnings you get. It is crucial to stay on designated boardwalks, as these "pre-play" signs can be followed by a sudden, violent release of scalding water and steam that reaches hundreds of degrees Fahrenheit in an instant.