The Atacama Desert is an oasis for astronomers looking for a pure sky, but a real hell on earth for anyone who ventures there without the right equipment. This is because, in the oldest and most arid deserts in the world, things unfold somewhat differently. Rain, for example, a source of life, can become a deadly scourge in this hyper-arid region.When the rain becomes deadlyAccording to scientists, the heart of Atacama has remained virtually arid for 15 million years, and history seems to indicate that no significant rain fell during the last 500 years. All this was turned upside down when in March and August 2015, then in June 2018, the desert experienced several extremely rare rain events.While for many the arrival of the rain would be considered beneficial, for the organisms adapted to the hyper arid climate of Atacama, these events were catastrophic. "When the rain arrived in Atacama, we hoped to see the desert come to life," recalls exobiologist Alberto Fairén. "Instead, we discovered the opposite, when the rain caused a massive extinction of microbial species indigenous to the hyper-arid desert."From the desert to MarsBecause of its extreme conditions, the desert is often used as a model for studying living conditions on Mars. A soil sample taken from the heart of the desert revealed the presence of 16 different microbial species. A number that may seem ridiculous compared to the population that would be found in a forest or even in the city, but it highlights the severity of 75 % to 87 % extinction measured after the rainy episodes."After the rain, only two to four species of microbes could be found in the lagoons," says Fairen. "Our results [published in Scientific Reports] show for the first time that providing large amounts of water to microorganisms - beautifully adapted to extract moisture almost absent from the driest environments - will kill them following osmotic shock", that is, when the concentrations change around a cell and change its permeability, causing stress.Many species have evolved to respond to this cellular stress. But in the hyper arid environment of Atacama, the need was never required. This study leads to an interesting conclusion by researchers: "[it] suggests that the recurrence of liquid water on Mars could have participated in the disappearance of Martian life, if it ever existed, instead of representing an opportunity for the resilient microbes to flourish again."