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This is Lake Castrovalva, located inside the Lechuguilla Cave in New Mexico. When Lechuguilla was fully opened up for exploration in 1986, a rare variety of speleothems – mineral formations – was discovered, including the rimstone dams pictured. “They are formed at the air/ water interface,” says microbiologist Dr Hazel Barton, who studied these caves. “When water splooshes over the edge, it changes the partial pressure of the dissolved CO2 in the water, causing it to off-gas. This causes the pH to change, which makes the calcium carbonate precipitate out so it adds itself to the edges. Based on where they are and how thick they are, I expect they took at least 500,000 years to form. Lake Castrovalva is a pretty special place. I’ve maybe only seen one or two other places where they form to this extent.”
The caves are home to several species of previously unknown microorganism, including some that may have medicinal properties – and one that, despite being four million years old, is resistant to modern antibiotics.