Tallest acidic geyser

Acidic hydrothermal features are distinguished by their low pH levels, which can be corrosive enough to erode the rock from which they emerge. More commonly associated with fumaroles (steam vents) and mudpots, the largest-known acidic geyser is Echinus Geyser in Yellowstone National Park, Wyoming, USA, which has waters of pH 3–4 (nearly the same as vinegar). Although currently in a period of relative dormancy, with months or even years between eruptions, in the past its jets have been reported to reach as much as 125 feet (38.1 metres) high, though more typically peaking at 75 feet (22.9 metres).

Echinus is so-named for the spiny-looking pebbles around its cone, where the acidic water has left sinter deposits that bear a resemblance to echinoderms such as sea urchins.

For many decades, Echinus was one of the most reliably regular geysers at Yellowstone, however its eruptions have become far more sporadic since the late 1990s. At its peak, it erupted every 20 to 80 minutes and eruptions lasted from a minute to over an hour (5–15 minutes was the most common).

Yellowstone National Park boasts the highest concentration of geysers (around 500) on Earth, which are among some 10,000 geothermal features there in total. It's also home to the world's tallest active geyser – Steamboat Geyser (which has neutral-alkaline water, as with most geysers) – which often reaches in excess of 90 m (295 ft).