When superheated water, rising under pressure from areas of molten rock beneath the ground, bursts through cracks and crevices in Earth, sending water and steam skyward, the eruption is called a geyser. When water from Earth’s great depths, heated above its boiling point but under too much pressure to turn to steam, encounters cooler surface water, the eruption is sometimes a violent gush. These conditions—intense heat generated underground from volcanic activity, water under pressure, and surface springs—must all be present to sustain Earth’s thousand or so geysers. A geyser might be considered the half-sister of an artesian well; the tight necks of its containment keep the water from rising easily and provide limited avenues of escape. When a geyser shoots skyward, it is usually at regular intervals. Geysers are as restless and changing as other parts of Earth. Occasionally new ones erupt, sometimes in quiet forests, killing trees. More than half of Earth’s geysers are found in Yellowstone Park.