Set against the back drop of awe inspiring geological beauty, Horizon follows a strange scientific adventure that sets out to discover how a mineral clad cave network - the height of a 30 storey building and the length of six football fields - came to exist deep below the Guadalupe Mountains in North America. But this journey soon unravels a multitude of inexplicable phenomena and obscure geological formations, leading to the discovery of extreme rock-eating microbes - a testimony from primordial Earth and a glimpse of life elsewhere in the Solar System. Acid attack Geologists believed that all limestone caves were formed by rain and underground water percolating through cracks in the rocks. Absorbing carbon dioxide from the soil, this water becomes weak carbonic acid, nibbling away at limestone, etching out networks of subterranean caves. However, the intricate cave structures beneath the Guadalupe Mountains in the Carlsbad Caverns of New Mexico are coated in glistening white, gypsum-clad walls. 400m under a desert, the world's largest gypsum chandeliers adorn a cavern called Lechuguilla Cave. There, dazzling white crystals create delicate branches up to 6m in length. Gypsum is soluble in water, so the 'water flow' theory doesn't fit here. Gypsum has been left copiously encrusting the walls, when it should have been dissolved in the cave formation process. Teams of scientists from the University of New Mexico, Portland State University and Chapman University discovered how such vast mineral coated caverns formed, and their explanation involved the work of a much stronger acid. Life in the dark The team visits a more active and dangerous cavern in South Mexico, searching for evidence of a cave in the act of formation. Respirators and poison-gas monitors are required for protection from the hydrogen sulphide gas and lethal sulphuric acid deep inside the Cueva de Villa Luz cave. Yet within this noxious environment life thrives. Microbes, spiders, insects, crabs, and fish all flourish in the complete darkness and caustic atmosphere. The team also discover 'snottites', mucous-like stalactites of sulphur-eating bacteria that drip sulphuric acid onto the surrounding limestone. These so-called 'extremophiles' are organisms that thrive in conditions that we consider unusual. They live in environments devoid of sunlight or oxygen, deep below the surface of the Earth. They tolerate high and low temperatures, extreme acidity and pressures that would crush surface creatures. They can survive at temperatures of 83°C in the bubbling hot springs at Yellowstone National Park. Colonies clump together in thick mats thriving by volcanic hydrothermal vents, belching out minerals and chemicals in boiling temperatures, deep within the ocean floor. It is these extreme microbes, feeding on oil far beneath the Carlsbad caves, or reacting in Cueva de Villa Luz that produce hydrogen sulphide. The gas emerges into the caves, where it reacts with oxygen to produce powerful sulphuric acid. This acid dissolves limestone eight times the volume of its weaker cousin, carbonic acid. It also leaves a mineral residue of gypsum. Other planets These microbial engineers have been busy hundreds of metres below the surface of the Earth for millennia. They have created geological beauty through biological activity. This process, still ongoing in Cueva de Villa Luz, was completed millions of years ago in Carlsbad and Lechuguilla. These environments are as extreme as the primordial Earth and may even be present under the freezing permafrost of the UV-saturated surface of Mars, or beneath the thick ice of Jupiter's moon, Europa.moreless
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