Acid Pools With a pH Below Zero - and Mars

In the Danakil Depression of northern Ethiopia, there is a place that looks like it belongs on another planet. The Dallol hydrothermal field glows in shades of neon yellow, acid green, gold and orange - vivid colors produced not by living things but by sulfates, iron oxides and copper salts baking in 108-degree springs.

pH Below Zero

The springs discharge water with a pH below zero - some pools measured as low as -1.7, making them more corrosive than battery acid and among the most hostile natural water environments ever recorded. They are also nearly ten times saltier than seawater and hotter than boiling point.

A Place Where Water Does Not Mean Life

You might expect something to live there. Life has been found in scalding deep-sea vents, in frozen Antarctic lakes, in nuclear reactor cooling pools. But a 2019 study led by biologist Purificacion Lopez Garcia, published in Nature Ecology and Evolution, concluded that nothing survives in Dallol's multi-extreme pools. The combination of hyper-acidity, hyper-salinity, extreme heat and magnesium-rich brine appears to cross every threshold at once. A separate team did find evidence of ultra-small archaea in nearby salt chimneys at slightly less extreme conditions - so the scientific debate continues - but the main acid pools appear sterile.

The finding matters beyond the strange beauty of the springs. If liquid water is not enough to sustain life, then the search for life elsewhere in the solar system becomes far more nuanced.

The Closest Thing on Earth to Ancient Mars

That is exactly why astrobiologists keep coming back. Researchers affiliated with NASA's astrobiology program have identified Dallol as a planetary field analog - an environment on Earth that mirrors what Mars may have looked like billions of years ago, when it had volcanic hydrothermal activity and acidic sulfate deposits. The Spirit rover explored a site on Mars called Gusev Crater with similar geochemical signatures. What scientists learn at Dallol helps shape where they look on the red planet.

The Hottest Inhabited Place - A Separate Record

Dallol sits inside the wider Danakil Depression, which holds a different record: the highest average annual temperature for an inhabited location on Earth, roughly 34.6 degrees Celsius, recorded between 1960 and 1966. The Afar people have lived and worked this landscape for centuries, harvesting salt from its vast evaporite flats. The neon acid pools are a geological feature within that broader world - extreme even by the standards of an already extreme place.